Handle missing less imports correctly

If a less import file was missing, this should be handled by calling the
callback passed to the imported with an error.  The old code just threw
an exception, and the result of this was that the code apparently
compiled fine, but the output file just contained
org.mozilla.javascript.Undefined@...
Merge pull request #2116 from jroper/less-missing-imports

Handle missing less imports correctly
Merge remote-tracking branch: upstream/master