gradle plugin: Add ignoreFailures flag to check task

This allows the user to configure a project with:

    check.ignoreFailures = true

to not fail the build if the check task fails.

See https://github.com/bndtools/bnd/issues/719

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #721 from bjhargrave/issue719

gradle plugin: Add ignoreFailures flag to check task