Added error description message for missing GAIA_KEYBOARD_LAYOUTS

Bug 946688 - Including a non-existent keyboard during make fails with undescriptive error  r=reviews.
bug 946730 - [Dialer] Do not show "xxx left" when hanging up the whole conference call
Merge pull request #14792 from brijeshks/nmaster

Added error description message for missing GAIA_KEYBOARD_LAYOUTS. r=yurenju
Merge pull request #14740 from rexboy7/fix-bug946730

bug 946730 - [Dialer] Do not show "xxx left" when hanging up the whole group call. r=etienne
Revert "Bug 945553 - Rewrite install-gaia.py to install-gaia.js"

This reverts commit b9129ad69a9ea91fda0c3f4c626c2f2eaf15d230.
Merge pull request #14876 from KevinGrandon/revert_bug_945553

Revert "Bug 945553 - Rewrite install-gaia.py to install-gaia.js"