_includes: better error on including directory

Previously, a missing include would result in an error like this:

    mydir is a directory. bst command expects a .bst file.

Note that the file containing the include was not mentioned.

Now we get an error like this instead:

    element.bst [line 12 column 0]: Include block references a
    directory instead of a file: 'mydir'.

buildstream/_includes.py

@@ -58,6 +58,10 @@ class Includes:
                         message = "{}: Include block references a file that could not be found: '{}'.".format(
                             include_provenance, include)
                         raise LoadError(LoadErrorReason.MISSING_FILE, message) from e
+                    elif e.reason == LoadErrorReason.LOADING_DIRECTORY:
+                        message = "{}: Include block references a directory instead of a file: '{}'.".format(
+                            include_provenance, include)
+                        raise LoadError(LoadErrorReason.LOADING_DIRECTORY, message) from e
                     else:
                         raise