Error messages for spawned processes don't include exit status and/or stdout

[alexcrichton]

🐛 Bug description

One issue we've run into with Cargo a lot is that processes inevitably fail for really weird reasons under really weird conditions. Currently it looks like wasm-pack will ferry along the stderr of a spawned process if the exit status doesn't indicate success, but this can often be confusing in situations such as:

• If the binary segfaulted (for whatever reason) often no output is produced, so it'll look like a failure with no output.
• Sometimes commands are buggy and send error information to stdout instead of stderr, also looking like a failure with no output.

Cargo's got some helpers for conveniently executing processes and wrapping it all up in an error if the process isn't successful while forwarding along all the relevant information necessary to display about the failed process.

I think that a good UI can also be preserved by perhaps doing something like whitelisting exit statuses to "if it exits with this then we don't need to display this as we know stderr contains useful information" (or something like that)

[fitzgen]

Although #387 fixed one instance of this, I think we want to leave this open for a blanket solution for all the child processes that we spawn.

[fitzgen]

Done now that #392 landed.