installer: Run the LESS compiler for *.less files

Includes the following improvements:

* Use the ResourceLoader module definitions (with a hardcoded list of
  module names) instead of a hardcoded list of filenames.
* Make the errors more discoverable like we do in load.php, prepending
  them instead of burying them in the middle somewhere.

Bug: 55589
Change-Id: Iab78a60209ab46a10d4c492c75527e717f36f803
Merge "installer: Run the LESS compiler for *.less files"