Take a little more care in set_backtrace().

Take a little more care in set_backtrace().

Coverity complained that the "errtrace" string is leaked if we return
early because backtrace_symbols fails.  Another criticism that could
be leveled at this is that not providing any hint of what happened is
user-unfriendly.  Fix that.

The odds of a leak here are small, and typically it wouldn't matter
anyway since the leak will be in ErrorContext which will soon get
reset.  So I'm not feeling a need to back-patch.