Fix SF bug #976608, Unhelpful error message when mtime of a module is -1

Will backport.

Misc/NEWS

@@ -12,6 +12,8 @@ What's New in Python 2.5 alpha 1?
 Core and builtins
 -----------------
 
+- SF Bug #976608: fix SystemError when mtime of an imported file is -1.
+
 - SF Bug #887946: fix segfault when redirecting stdin from a directory.
   Provide a warning when a directory is passed on the command line.
 

Python/import.c

@@ -868,8 +868,12 @@ load_source_module(char *name, char *pathname, FILE *fp)
 	PyObject *m;
 
 	mtime = PyOS_GetLastModificationTime(pathname, fp);
-	if (mtime == (time_t)(-1))
+	if (mtime == (time_t)(-1)) {
+		PyErr_Format(PyExc_RuntimeError,
+			     "unable to get modification time from '%s'",
+			     pathname);
 		return NULL;
+	}
 #if SIZEOF_TIME_T > 4
 	/* Python's .pyc timestamp handling presumes that the timestamp fits
 	   in 4 bytes. This will be fine until sometime in the year 2038,