Unexpected panics should still print the full panic message

[alexcrichton]

Unexpected panics are unfortunate but they're currently covered up and written out to an auxiliary file. This makes panics in CI difficult to debug, especially at a glance, as CI builders are likely not uploading those files.

wasm-pack should still print something to the console indicating what sort of error just happened.

[drager]

I will start working on this.