Make sure it consistently uses stderr, rather than a mixture of stdout
and stderr. Also rename it to dieOnBuildFailure to make it clear that
it is fatal in the case of build failures.

In general we report build failures in two steps: header plus build log
for each failing package, then a final die exception with summary.
Previously the build log step was reported to stdout whereas the die
exception message gets reported to stderr. So we switch that to use
new dieMsg and dieMsgNoWrap utils so that it all goes to stderr. Also,
like die, these are reported irrespective of the verbosity.

This is more or less just a workaround for the fact that we do not yet
have a nice structured/formatted error mechanism that would allow us to
throw just the one error in this case.