[Doitsu]

@jackie_w: There appears to be a bug in the plugin, because the error message should only be displayed, if epubcheck didn't report a file name.
Also file names should be displayed without path information. For an example, have a look at the attached screenshot, which was generated by running the Linux version of the Calibre epubcheck plugin.
Most likely the structure of your epub is different from the structure of my test file or it's an OS-specific bug. Can you please test my test file on your Linux/macOS(?) machine?
If it works as designed, please PM me a link to the file that caused the bug or another Public Domain file that causes the same bug.
Alternatively, saving the epub in Calibre Editor before running the epubcheck plugin might also take care of this problem.