Build fails if api is not dumped

[altavir]

The build fails with non-informative error (directory not found) if the initial API dump is not done before the build.
I think that the default behavior should be to give a warning and ignore the plugin altogether.

[qwwdfsad]

Skipping the validation as the default behaviour for an applied plugin is a no-go. If skipping is the desired behaviour, the plugin shouldn't have been applied in the first place.

Added clear exception message instead