Cleanup output if version mismatch.

[frewsxcv]

If there's a version mismatch, this is what is outputted:

process didn't exit successfully: `/Users/corey/dev/targets/fuzzer-honggfuzz/hfuzz_target/release/build/honggfuzz-32eedb7c8be2fdad/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'hongfuzz dependency (0.5.19) and build command (0.5.7) versions do not match', /Users/corey/.cargo/registry/src/github.com-1ecc6299db9ec823/honggfuzz-0.5.19/build.rs:24:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Seems like we can just print the error instead of doing the panic message dance.

[PaulGrandperrin]

Thanks! bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push