Save more descriptive error msg from FBS/BKS, relay it up to lldb.

When lldb requests an app launch through FrontBoard/BackBoard,
we get back an NSError object if there was a problem with an
integer error code and a descriptive text string.  debugserver
would log the descriptive text string to the console, but it
would only save the error code value, ask for the
much-less-specific name of that error code, and send that very
generic error word back to lldb.

This patch saves the longer description of the failure when
available, and sends that to lldb.  If unavailable, it falls
back to sending up the generic description of the error code
as it was doing before.

This only impacts the iOS on-device debugserver.

<rdar://problem/49953304>