cmake: Fix --build crash on bad CMAKE_GENERATOR in cache

If we fail to create the generator named by CMAKE_GENERATOR, exit
with an error message instead of crashing.  While at it, fix the
wording of the error message when CMAKE_GENERATOR is not set.

Extend the RunCMake.CommandLine test with cases covering the
"cmake --build" option when the named directory does not provide
a CMakeCache.txt with a valid CMAKE_GENERATOR.
Merge branch 'fix-build-crash-on-bad-generator' into release