Extend parsed but not stored message

During parsing a TYPO3 mailing list some messages occurred:

```
...
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2003-December.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-January.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-February.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-March.txt.gz
   ***WARNING: 8 messages (out of 496) parsed but not stored***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-April.txt.gz
   ***WARNING: 8 messages (out of 732) parsed but not stored***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-May.txt.gz
   ***WARNING: 33 messages (out of 770) parsed but not stored***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-June.txt.gz
   ***WARNING: 8 messages (out of 739) parsed but not stored***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-July.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-August.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-September.txt.gz
...
```

At this time i asked me "Why does this messages not stored?"

So i added a reason to the error message.
The message now looks like:

```
...
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-February.txt.gz
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-March.txt.gz
   ***WARNING: 8 messages (out of 496) parsed but not stored (8 duplicate, 0 runtime error)***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-April.txt.gz
   ***WARNING: 8 messages (out of 732) parsed but not stored (8 duplicate, 0 runtime error)***
Analyzing /home/vagrant/.mlstats/compressed/lists.typo3.org/pipermail/typo3-dev/2004-May.txt.gz
...
```
Can we avoid *magic numbers*?

So, instead of returning -1, -2, ... we can return a constant or enum.