Less errors should be written to stderr

[tomgreenfield]

Subject of the issue/enhancement/features

Less errors should be written to stderr to enable reliable error handling in the authoring tool.

Your environment

• Framework 5.11
• Authoring tool 0.11 (unreleased)

Steps to reproduce

1. Add a reference to a missing variable in a Less file
2. grunt dev

Expected behaviour

Less error should be written to stderr.

Actual behaviour

stderr is empty meaning stdout has to be parsed manually to find the fatal error.