twister: bugfix: Make BuildError exception cause test to report error

When no/too many elf files are detected after a build a BuildError
exception is raised. However, it was not being counted as an issue
with a test. With the patch satuses of tests' with such exception
are reported as errors. Whithout it, twister finished without
reported errors and was getting green CI checks.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>