Add info to error message.

Add info to the error message when the configuration file could not be found.
Suggests that the user verify they ran from the root of the source tree.

Testing:
    make test - python2/3 - pass, 1 skip
    manual testing - escomp/cesm, clm-demo - ok

Author: bandre-ucar

manic/externals_description.py

@@ -71,7 +71,8 @@ def read_externals_description_file(root_dir, file_name):
     file_path = os.path.join(root_dir, file_name)
     if not os.path.exists(file_name):
         msg = ('ERROR: Model description file, "{0}", does not '
-               'exist at {1}'.format(file_name, file_path))
+               'exist at path:\n    {1}\nDid you run from the root of '
+               'the source tree?'.format(file_name, file_path))
         fatal_error(msg)
 
     externals_description = None