Previously, if the tests didn't pass, the check phase would evaluate to #f,
but the package would be built sucessfully. This changes all the phases to
raise exceptions if errors are encountered, and return #t otherwise.
This involves using invoke rather than system*, so that exceptions are raised
if the program exits with a status other than 0, and also returning #t at the
end of functions.
* gnu/build/ruby-build-system.scm (unpack): Use invoke rather than system*,
and return #t at the end.
(build, check): Use invoke rather than system*.
(install): Remove the use of "and", and rewrite the error handling to raise an
exception.
(wrap): Return #t.
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