Merge pull request #1440 from jedwards4b/better_msg_for_config_files_…

…not_found

improve error message when config_files.xml is not found

If CIME_MODEL is set incorrectly the current error message from scripts_regression_tests.py is obscure, this error somewhat clarifies the problem.
Test suite: hand testing
Test baseline:
Test namelist changes:
Test status: bit for bit

Fixes

User interface changes?:

Code review:

scripts/lib/CIME/XML/files.py

@@ -20,6 +20,7 @@ def __init__(self):
         """
         cimeroot = get_cime_root()
         infile = os.path.join(cimeroot, "config", get_model(), "config_files.xml")
+        expect(os.path.isfile(infile), "Could not find or open file %s"%infile)
         schema = os.path.join(cimeroot, "config", "xml_schemas", "entry_id.xsd")
         EntryID.__init__(self, infile, schema=schema)