loader.py: Check whether the dir specified is also a <dir>.bst file

  - Slight modification to the wording of existing error message.
  - Closes #446

buildstream/_loader/loader.py

@@ -229,6 +229,16 @@ class Loader():
                     detail = "Did you mean '{}'?".format(element_relpath)
                 raise LoadError(LoadErrorReason.MISSING_FILE,
                                 message, detail=detail) from e
+            elif e.reason == LoadErrorReason.LOADING_DIRECTORY:
+                # If a <directory>.bst file exists in the element path,
+                # let's suggest this as a plausible alternative.
+                message = str(e)
+                detail = None
+                if os.path.exists(os.path.join(self._basedir, filename + '.bst')):
+                    element_name = filename + '.bst'
+                    detail = "Did you mean '{}'?\n".format(element_name)
+                raise LoadError(LoadErrorReason.LOADING_DIRECTORY,
+                                message, detail=detail) from e
             else:
                 raise
         self._options.process_node(node)

buildstream/_yaml.py

@@ -186,7 +186,7 @@ def load(filename, shortname=None, copy_tree=False):
                         "Could not find file at {}".format(filename)) from e
     except IsADirectoryError as e:
         raise LoadError(LoadErrorReason.LOADING_DIRECTORY,
-                        "{} is a directory. bst command is expecting a .bst file"
+                        "{} is a directory. bst command expects a .bst file."
                         .format(filename)) from e