Return full error message from xmllint

[peverwhee]

Update call_command function to include stderr output in CCPPError message.

Right now, when xmllint-ing is done via xml_tools.py (validate_xml_file),
the output of the linter is not returned. This PR adds the stderr output
(if any) to the returned error message.

PR also decodes error messages for cleaner output.

User interface changes?: No

Testing:
test removed: N/A
unit tests: Added new doctest to test error message
system tests: N/A
manual testing: Ran with/parsed new error messages within CAM-SIMA

[climbfuji]

Looks good to me. Thanks for fixing this

[gold2718]

The output looks incorrect if a command has output to both stdout and stderr.

[gold2718]

Looks great now, thanks!

[dustinswales]

@peverwhee These changes look good to me. Thanks!