MFC r325116,r325515: Stop masking errors during buildenv

MFC r325116(imp): Return proper status from buildenv.

make buildenv BUILDENV_SHELL=<some command> more useful. Remove '||
true' from the command line so that errors are properly
returned. There appears to be no reason for it, and it dates back to
the original commit by ru@.

MFC r325515 (imp): Note interactive shell errors for make buildenv and add a
warning for people tempted to add back the || true to get rid of them.