Improve error message in AbstractFileSource#assertFilePathIsUnderRoot()

[oleg-nenashev]

As reported on Slack, the current error message in the response is confusing and does not contain a reason when a non-absolute path is passed through API

<title>Error 500 com.github.tomakehurst.wiremock.security.NotAuthorisedException: Access to file
        ../resources/__files/get-users-200.json is not permitted</title>
getting this error when i am trying to connect to my json file.

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• The PR request is well described and justified, including the body and the references
• The PR title represents the desired changelog entry
• The repository's code style is followed (see the contributing guide)
• Test coverage that demonstrates that the change works as expected => SingleRootFileSourceTest#writeThrowsExceptionWhenRootIsNotDir()
• [ x For new features, there's necessary documentation in this pull request or in a subsequent PR to wiremock.org

[oleg-nenashev]

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