Turns out we have a surprising number of catch (...) clauses in the viewer
code base. If all we currently do is

    LL_ERRS() << "unknown exception" << LL_ENDL;

then call CRASH_ON_UNHANDLED_EXCEPTION() instead. If what we do is

    LL_WARNS() << "unknown exception" << LL_ENDL;

then call LOG_UNHANDLED_EXCEPTION() instead.

Since many places need LOG_UNHANDLED_EXCEPTION() and nobody catches
LLContinueError yet, eliminate LLContinueError& parameter from
LOG_UNHANDLED_EXCEPTION(). This permits us to use the same log message as
CRASH_ON_UNHANDLED_EXCEPTION(), just with a different severity level.

Where a catch (...) clause actually provides contextual information, or makes
an error string, add boost::current_exception_diagnostic_information() to try
to figure out actual exception class and message.