Improve error message when the apiCheck fails due to missing klib dump file

[fzhinkin]

Currently, when there's no klib.api file in the project, the apiCheck task fails with an error message complaining about missing the intermediate file, not the source klib.api file that has to be placed in the repo:

* What went wrong:
Execution failed for task ':redwood-layout-modifiers:klibApiCheck'.
> Expected folder with API declarations '/private/tmp/bugs/redwood/redwood-layout-modifiers/build/api/klib-all/extracted' does not exist.
  Please ensure that ':apiDump' was executed in order to get API dump to compare the build against

It would be nice to point users to a missing from the projects source directory, not to some obscure intermediate file.

[fzhinkin]

After the #204 the error message should look much better:

* What went wrong:
Execution failed for task ':klibApiExtractForValidation'.
> File with project's API declarations 'api/bcv-demo.klib.api' does not exist.
  Please ensure that ':apiDump' was executed in order to get API dump to compare the build against