boot: print reasonable stacktrace for bugs

A recent patch aimed to remove the number of times a stack-trace is
printed if a bug is discovered during start-up.

Unfortunately, the patch resulted in only the outer-most (most recent)
RuntimeException being shown.  We wrap RuntimeExceptions inside
a RuntimeException to provide context-specific information (e.g., a
line number of a file).  Therefore, the most recent stack-trace isn't
interesting.

This patch fixes master so that the root cause is printed instead.

Target: master
Patch: http://rb.dcache.org/r/5958/
Acked-by: Albert Rossi
Requires-notes: no
Requires-book: no