bugpoint: add flag -verbose-errors

The default behavior of bugpoint is to print "<crash>" when it finds a reduced
test that crashes compilation.  With this flag we now can see the output of the
crashing program.  This is useful to make sure it is the same error being
tracked down and not a different error that happens to crash the compiler as
well.

Differential Revision: https://reviews.llvm.org/D22411

llvm-svn: 275646