Show main exception only with --verbose

We call websockify from our web app and users often hit used port. In that case
websockify exceptions out with strack trace which causes ABRT (Bug Reporting
Tool) to collect exception information, system information and making a report.

This patch prints error messages by default to stderr and only if --verbose is
used, the full stacktrace is shown. I can introduce another option like --trace
if you do not like this one.