throw (cargo) error on version mismatch

[Lotterleben]

resolves #187 , throws

error: failed to select a version for `defmt`.
    ... required by package `my-app v0.1.0 (/Users/lottesteenbrink/ferrous/knurling/my-app)`
    ... which is depended on by `testsuite v0.1.0 (/Users/lottesteenbrink/ferrous/knurling/my-app/testsuite)`
versions that meet the requirements `=0.1.0` are: 0.1.0

the package `defmt` links to the native library `defmt`, but it conflicts with a previous package which links to `defmt` as well:
package `defmt v0.24.0 (/Users/lottesteenbrink/ferrous/knurling/defm_versionbreak)`
    ... which is depended on by `testsuite v0.1.0 (/Users/lottesteenbrink/ferrous/knurling/my-app/testsuite)`

failed to select a version for `defmt` which could resolve this conflict
The terminal process "/bin/zsh '-c', 'cargo test --package testsuite --test test'" failed to launch (exit code: 101).

[japaric]

Nice!