surface build error content through test event system

Summary:
print build errors nicely in test output

This test infrastructure change adds a new Python exception
for test subject builds that fail.  The output of the build
command is captured and propagated to both the textual test
output display code and to the test event system.

The ResultsFormatter objects have been modified to do something
more useful with this information.  The xUnit formatter
now replaces the non-informative Python build error stacktrace
with the build error content.  The curses ResultsFormatter
prints a 'B' for build errors rather than 'E'.

The xUnit output, in particular, makes it much easier for
developers to track down test subject build errors that cause
test failures when reports come in from CI.

Reviewers: granata.enrico

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20252

llvm-svn: 269525