After #666, incorrect error messages were shown for failures that
occurred during buildpack compilation packaging, since the
handling in `TestRunner::build_internal` used the wrong message
text, plus didn't include the error struct in the `panic!` string.

These have been fixed, plus display representations added to
all of the new error variants.

In addition, the usages of `assert!` and `.unwrap()` in functions that
return `Result` have been replaced with new error enum variants.

The change in output can be seen in the new tests added by #717.

There is one last scenario that doesn't yet have a test - the testing
of the cross-compilation output itself. However, I'll add that separately,
since it's going to be more complex to test / there are a few different
options that we'll need to choose between.

Fixes #704.
Fixes #709.
Fixes #710.
GUS-W-14395170.