FIX: Removed plain string messages:

[carlwilson]

• replaced all plain string instatiations of ErrorMessage and InfoMessage with JhoveMessage instances;
• PNG Module:
  • tidied info.setMessage() calls;
  • use boolean flag for chunk state checking;
• GZIP & WARC Module:
  • use new Jwatt/Jhove ID minter to derive message IDs from the diagnosis type;
  • removed duplicate code that extracted messages from Jwatt diagnosis;
  • message resources and JhoveMessage constants to MessageConstants class;
  • generated codes to start at 100 to avoid clashes with any module string codes;
• EPUB Module:
  • minor warnings and a single error instatiation;
• addressed minor warnings; and
• removed commented out code.

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ef1d25e) 46.22% compared to head (2c43e54) 46.22%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## integration #836 +/- ## ============================================== Coverage 46.22% 46.22% Complexity 1056 1056 ============================================== Files 57 57 Lines 9057 9057 Branches 1607 1607 ============================================== Hits 4187 4187 Misses 4330 4330 Partials 540 540 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=openpreserve)

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.