InternalPushConnection: don't swallow RuntimeException

InternalPushConnection: don't swallow RuntimeException

Uncaught exceptions are handled by java.lang.Thread's handler, which
prints it to stderr.

This is useful because InternalPushConnection is used in tests, and
during development, the server side may have programming errors that
manifest as RuntimeExceptions.

Before this change, all types of failures would lead to a uniform
failure message "test://test/conn0: push not permitted" on the client.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I0983cbeb86d36fa7a9313373f5fce54971f804ec