Fix the error message on hg missing but asked for
Merge pull request #5 from tomassedovic/patch-1

Fix the error message when hg isn't installed
Bunch of fixes
Better error reporting on failed `build_cmd` run

When running the custom build command fails, this prints out a correct error
message, return code, stdout and stderr.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	cargo-lite.py