Clarify the intention of raising ImportError for importlib.abc.PyLoader.(source|bytecode)_path.

Doc/library/importlib.rst

@@ -230,7 +230,7 @@ are also provided to help in implementing the core ABCs.
 
         An abstract method that returns the path to the source code for a
         module. Should return :keyword:`None` if there is no source code or
-        raise :exc:`ImportError` if the module cannot be found.
+        raise :exc:`ImportError` if the module cannot be handled by the loader.
 
     .. method:: get_filename(fullname)
 
@@ -282,7 +282,8 @@ are also provided to help in implementing the core ABCs.
         An abstract method which returns the path to the bytecode for the
         specified module, if it exists. It returns :keyword:`None`
         if no bytecode exists (yet).
-        Raises :exc:`ImportError` if the module is not found.
+        Raises :exc:`ImportError` if the module cannot be handled by the
+        loader.
 
     .. method:: get_filename(fullname)