Make MD5 mismatch error message more user-friendly

As a user, I want to understand what an MD5 mismatch during openbook download means so that I know what to do next.

Acceptance criteria:

Hide callstack, replace with meaningful error message
Explain possible reasons for MD5 mismatch:
- corrupt download archive, e.g. due to interrupted download
- changed archive on server with possible incompatibilities during the cleaning process
Explain options for next steps:
- restart program with downloaded archive and see if it is unpacked and cleaned correctly anyway
- delete archive after incomplete download and restart program in order to check if the error occurs again
- use --write-config option and update MD5 for corresponding openbook manually to avoid future error messages
Consider adding a CLI switch for a processing mode handling MD5 mismatches as warnings instead of errors and just continue processing.

Related tickets: #4, #13, #14, #24

Aktuelle Entwicklerversion (Snapshot) mit ausführlicherer Fehlermeldung kann heruntergeladen werden unter http://scrum-master.de/download/GalileoOpenbookCleaner/openbook_cleaner-1.2.0-SNAPSHOT.jar.

Fehlermeldung sieht nun so aus:

Possible download problem detected: MD5 checksum mismatch.
  Book title:    Java 7 - mehr als eine Insel
  Download file: ..\openbooks\galileocomputing_javainsel.zip
  MD5 expected:  fffffa7cba1f2f9573663f37f711c9c
  MD5 actual:    6f6aea7cba1f2f9573663f37f711c9c

Possible reasons are:
  - corrupt download file due to incomplete download process
  - download file was updated on server

The program will try to unpack and clean the download archive anyway.

In case of problems you have the following options:
  - If you think the download file is corrupt, just delete it and restart the
    program so as to get a fresh download.
  - If you think that the download file is okay but the cleanup does not work
    correctly, please notify the program author by creating a ticket at
    https://github.com/kriegaex/Galileo-Openbook-Cleaner/issues, but only if
    an equivalent ticket does not already exist.
  - If the cleanup works correctly even despite the MD5 mismatch, please also
    notify the author as explained above. Meanwhile you can avoid this error
    message by using the --write-config option first and afterwards update the
    MD5 checksum for the corresponding openbook manually in config.xml.

kriegaex / Galileo-Openbook-Cleaner

Make MD5 mismatch error message more user-friendly #25