Cli tools currently catch all exceptions, and only print the exception
message, except when a special system property is set. Even with this
flag set, certain exceptions, like IOException, are captured and their
stack trace is always lost.
This change adds a UserError class, which can be used a cli tools to
specify a message to the user, as well as an exit status. All other
exceptions are propagated out of main, so java will exit with non-zero
and print the stack trace.
a2c37c0989