Trace JavaScript exceptions instead of hard-printing them.

Hence the traces are 'suppressed' by sbt and can be
recalled with the 'last' command.
Throw an exception when at least one test failed.

This causes an invocation to `sbt scalajs-test/test` to
actually return an exit code of 1 if a test fails.
This in turn will cause the Travis build to fail if
there is at least one test failing.