teach packstack about "could not autoload" errors

If a Puppet custom type/provder/etc fails to load because of missing
dependencies, Puppet will log an error of the form:

Could not autoload <thing>: Could not autoload </path/to/module.rb>: no such file to load -- rubygems at
</path/to/manifest.pp>:<linenumber> on node <hostname>

This change teaches packstack to recognize any messages starting with
'Could not autoload' as errors.

Change-Id: I3e0cb7f5e8871cfdc122dfea3adcb2bb7a8d31d1
Closes-bug: 1312224