recover from panic when generating chunks

[sbfaulkner]

further to #1803 this PR recovers from any panics in the goroutines, converting them to errors (which can then be handled appropriately)

eg.

$ bin/esbuild --loader=css --minify <bad.css
 > <stdin>:9:1: warning: Expected ";"
    9 │ }
      ╵  ^

 > error: Internal error: runtime error: index out of range [1] with length 1

1 warning and 1 error

[sbfaulkner]

thanks @evanw - the changes you made on top of this make it even better 🥂

I'm not sure what your release cadence is, but I'd love to see a 0.14.1 when you get a chance ❤️