diff --git a/elixir/web.py b/elixir/web.py
index a579cd9..fcd9f10 100755
--- a/elixir/web.py
+++ b/elixir/web.py
@@ -80,17 +80,11 @@ class SourceResource:
 
         query = get_query(req.context.config.project_dir, project)
         if not query:
-            resp.status = falcon.HTTP_NOT_FOUND
-            resp.content_type = falcon.MEDIA_HTML
-            resp.text = get_error_page(req.context, "Unknown project")
-            return
+            raise falcon.HTTPNotFound('Error', 'Unknown project')
 
         # Check if path contains only allowed characters
         if not search('^[A-Za-z0-9_/.,+-]*$', path):
-            resp.status = falcon.HTTP_BAD_REQUEST
-            resp.content_type  = falcon.MEDIA_HTML
-            resp.text = get_error_page(req.context, "Path contains characters that are not allowed.")
-            return
+            raise falcon.HTTPBadRequest('Error', 'Path contains characters that are not allowed')
 
         if version == 'latest':
             version = parse.quote(query.query('latest'))
@@ -134,10 +128,7 @@ class IdentPostRedirectResource:
             post_family = 'C'
 
         if not post_ident:
-            resp.status = falcon.HTTP_BAD_REQUEST
-            resp.content_type = falcon.MEDIA_HTML
-            resp.text = get_error_page(req.context, "Invalid identifier")
-            return
+            raise falcon.HTTPBadRequest('Error', 'Invalid identifier')
 
         post_ident = post_ident.strip()
         resp.status = falcon.HTTP_MOVED_PERMANENTLY
@@ -150,10 +141,7 @@ class IdentResource(IdentPostRedirectResource):
     def on_get(self, req, resp, project, version, family, ident):
         query = get_query(req.context.config.project_dir, project)
         if not query:
-            resp.status = falcon.HTTP_NOT_FOUND
-            resp.content_type = falcon.MEDIA_HTML
-            resp.text = get_error_page(req.context, "Unknown project.")
-            return
+            raise falcon.HTTPNotFound('Error', 'Unknown project')
 
         if version == 'latest':
             version = parse.quote(query.query('latest'))
@@ -546,17 +534,35 @@ class RequestContextMiddleware:
     def process_request(self, req, resp):
         req.context = get_request_context(req.env)
 
+# Serialies caught exceptions to JSON or HTML
+# See https://falcon.readthedocs.io/en/stable/api/app.html#falcon.App.set_error_serializer
+def error_serializer(req, resp, exception):
+    preferred = req.client_prefers((falcon.MEDIA_HTML, falcon.MEDIA_JSON))
+
+    if preferred is not None:
+        if preferred == falcon.MEDIA_JSON:
+            resp.data = exception.to_json()
+            resp.content_type = falcon.MEDIA_JSON
+        else:
+            resp.text = get_error_page(req.context, exception.title, exception.description)
+            resp.content_type = falcon.MEDIA_HTML
+
+    resp.append_header('Vary', 'Accept')
+
 # Builds and returns the Falcon application
 def get_application():
     app = falcon.App(middleware=[
         RawPathComponent(),
         RequestContextMiddleware(),
     ])
+
     app.router_options.converters['project'] = ProjectConverter
     app.router_options.converters['version'] = VersionConverter
     app.router_options.converters['ident'] = IdentConverter
     app.router_options.converters['family'] = FamilyConverter
 
+    app.set_error_serializer(error_serializer)
+
     app.add_route('/{project:project}/{version:version}/source/{path:path}', SourceResource())
     app.add_route('/{project:project}/{version:version}/source', SourceWithoutPathResource())
     app.add_route('/{project:project}/{version:version}/ident', IdentPostRedirectResource())
diff --git a/elixir/web_utils.py b/elixir/web_utils.py
index c24f256..5d1caff 100644
--- a/elixir/web_utils.py
+++ b/elixir/web_utils.py
@@ -12,7 +12,10 @@ def validate_project(project):
 class ProjectConverter(falcon.routing.BaseConverter):
     def convert(self, value: str):
         value = parse.unquote(value)
-        return validate_project(value)
+        project = validate_project(value)
+        if project is None:
+            raise falcon.HTTPBadRequest('Error', 'Invalid project name')
+        return project
 
 def validate_version(version):
     if version is not None and re.match(r'^[a-zA-Z0-9_.,:/-]+$', version):
@@ -22,7 +25,10 @@ def validate_version(version):
 class VersionConverter(falcon.routing.BaseConverter):
     def convert(self, value: str):
         value = parse.unquote(value)
-        return validate_version(value)
+        version = validate_version(value)
+        if version is None:
+            raise falcon.HTTPBadRequest('Error', 'Invalid version name')
+        return version
 
 def validate_ident(ident):
     if ident is not None and re.match(r'^[A-Za-z0-9_,.+?#-]+$', ident):