Crash if metadata cannot be retrieved

[evnu]

This PR fixes #394 by crashing if cargo metadata fails. Example error:

== Compilation error in file lib/binary_example.ex ==
** (RuntimeError) calling `cargo metadata` failed.

    (rustler 0.22.2) lib/rustler/compiler/config.ex:87: Rustler.Compiler.Config.external_resources/2
    (rustler 0.22.2) lib/rustler/compiler/config.ex:69: Rustler.Compiler.Config.build/1
    (rustler 0.22.2) lib/rustler/compiler.ex:9: Rustler.Compiler.compile_crate/2
    lib/binary_example.ex:2: (module)

[evnu]

bfbd9cf (badmatch) and 0976bd8 (raise) are two different ways to fix #394. Badmatch is shorter, but does not result in a nice error message, raising is longer but the error message is better. I think the latter is preferable here.

[hansihe]

LGTM, raising is the correct behavior here

[evnu]

I changed the output format a little bit in c6492f6. Output: is misleading if no output is empty.