Make API http errors return JSON

Fixes #384

app/blueprints/api/__init__.py

@@ -14,8 +14,36 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+import json
 from flask import Blueprint
 
+from .support import error
+
 bp = Blueprint("api", __name__)
 
+
 from . import tokens, endpoints
+
+
+@bp.errorhandler(400)
+@bp.errorhandler(401)
+@bp.errorhandler(403)
+@bp.errorhandler(404)
+def handle_exception(e):
+	"""Return JSON instead of HTML for HTTP errors."""
+	# start with the correct headers and status code from the error
+	response = e.get_response()
+	# replace the body with JSON
+	response.data = json.dumps({
+		"success": False,
+		"code": e.code,
+		"name": e.name,
+		"description": e.description,
+	})
+	response.content_type = "application/json"
+	return response
+
+
+@bp.route("/api/<path:path>")
+def page_not_found(path):
+	error(404, "Endpoint or method not found")