Preventing traceback for detailed diagnostic

Previously we'd show a Python traceback if the user asked for a detailed
diagnostic in a file that wasn't supported by Clang (something written in Python
perhaps). Now we show an nice, far less scary message informing the user of
this.

Fixes #748.

python/ycm/completers/completer.py

@@ -27,6 +27,7 @@ else:
   from ycm_core import FilterAndSortCandidates
 
 from ycm.completers.completer_utils import TriggersForFiletype
+from ycm.server.responses import NoDiagnosticSupport
 
 NO_USER_COMMANDS = 'This completer does not define any commands.'
 
@@ -245,11 +246,11 @@ class Completer( object ):
 
 
   def GetDiagnosticsForCurrentFile( self, request_data ):
-    return []
+    raise NoDiagnosticSupport
 
 
   def GetDetailedDiagnostic( self, request_data ):
-    pass
+    raise NoDiagnosticSupport
 
 
   def _CurrentFiletype( self, filetypes ):

python/ycm/server/responses.py

@@ -28,6 +28,9 @@ NO_EXTRA_CONF_FILENAME_MESSAGE = ( 'No {0} file detected, so no compile flags '
   'are available. Thus no semantic support for C/C++/ObjC/ObjC++. Go READ THE '
   'DOCS *NOW*, DON\'T file a bug report.' ).format( YCM_EXTRA_CONF_FILENAME )
 
+NO_DIAGNOSTIC_SUPPORT_MESSAGE = ( 'YCM has no diagnostics support for this '
+  'filetype; refer to Syntastic docs if using Syntastic.')
+
 
 class ServerError( Exception ):
   def __init__( self, message ):
@@ -47,6 +50,11 @@ class NoExtraConfDetected( ServerError ):
       NO_EXTRA_CONF_FILENAME_MESSAGE )
 
 
+class NoDiagnosticSupport( ServerError ):
+  def __init__( self ):
+    super( NoDiagnosticSupport, self ).__init__( NO_DIAGNOSTIC_SUPPORT_MESSAGE )
+
+
 def BuildGoToResponse( filepath, line_num, column_num, description = None ):
   response = {
     'filepath': os.path.realpath( filepath ),

python/ycm/server/tests/diagnostics_test.py

@@ -21,11 +21,13 @@ from ..server_utils import SetUpPythonPath
 SetUpPythonPath()
 from .test_utils import Setup, BuildRequest
 from webtest import TestApp
-from nose.tools import with_setup
+from nose.tools import with_setup, eq_
 from hamcrest import ( assert_that, contains, contains_string, has_entries,
                        has_entry, empty )
+from ..responses import NoDiagnosticSupport
 from .. import handlers
 import bottle
+import httplib
 
 bottle.debug( True )
 
@@ -104,3 +106,19 @@ struct Foo {
   assert_that( results,
                has_entry( 'message', contains_string( "expected ';'" ) ) )
 
+
+@with_setup( Setup )
+def GetDetailedDiagnostic_JediCompleter_DoesntWork_test():
+  app = TestApp( handlers.app )
+  diag_data = BuildRequest( contents = "foo = 5",
+                            line_num = 1,
+                            filetype = 'python' )
+  response = app.post_json( '/detailed_diagnostic',
+                            diag_data,
+                            expect_errors = True )
+
+  eq_( response.status_code, httplib.INTERNAL_SERVER_ERROR )
+  assert_that( response.json,
+               has_entry( 'exception',
+                          has_entry( 'TYPE', NoDiagnosticSupport.__name__ ) ) )
+