Inconsistent warnings for missing externals attributes

[gold2718]

Summary of Issue:

Warning when protocol is missing is unclear

Expected behavior and actual behavior:

Expect: a warning similar to if the repository url is left out:

$ ~/Projects/manage_externals/checkout_externals --externals Externals2.cfg 
Processing externals description file : Externals2.cfg
ERROR:root:Model description is under specified! Must have "repo_url" in repo description for "gtools"

ERROR: Model description is under specified! Must have "repo_url" in repo description for "gtools"

Actual (if protocol line is omitted):

$ ~/Projects/manage_externals/checkout_externals                            
Processing externals description file : Externals.cfg
u'protocol'

Steps to reproduce the problem (should include model description file(s) or link to public repository):

Remove the protocol attribute line from an Externals.cfg entry, then run checkout_externals.

What is the changeset ID of the code, and the machine you are using: a48558d

have you modified the code? If so, it must be committed and available for testing: No

Screen output or log file showing the error message and context: See above