Rollup merge of rust-lang#31332 - durka:errorck-fileline, r=brson

Small modification to rust-lang#31288. Shows full path and line number for unused error codes.

Author: Manishearth

src/etc/errorck.py

@@ -114,7 +114,7 @@ def check_unused_error_codes(error_codes, check_error_codes, filenames, dirnames
         if errcode in errcode_checked:
             continue
         all_errors.append(errcode)
-        print("error: unused error code: " + errcode)
+        print("error: unused error code: {0} ({1}:{2})".format(*errcode_map[errcode][0]))
         errors = True