Replace backslashes in test error messages, they may contain windows filenames.
Add a newline to file not found messages.
Fixes #956
Closes #957
Merge pull request #957 from johnjbarton/windows-error-mismatch

    Replace backslashes in test error messages, they may contain windows filenames.
    Add a newline to file not found messages.
    Fixes #956
    Closes #957