Rename findNinja to findOrBuildNinja and fail early if gyp fails

We should not not use the $$ syntax to collect the output of the gyp
call instead we should just fail with an error in case gyp_qtwebengine
fails.

Change-Id: Ib6218bdfb998100a3717470713ed5bf5f30db339
Reviewed-by: Pierre Rossi <pierre.rossi@gmail.com>