Make version_from_string() more tolerant
Drop autodetection of hooks, but still extract version from hook name
Un-indent upgrade functions
_upgrade_hooks is now a dict, with version as key
Remove useless intermediate variable
Use exceptions in place of assertions -> VersionError
version_from_string(): match end of the string instead full string
Let version_from_string() raises an exception if hook name is incorrect
Mark messages for translation.
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Merge pull request #203 from zas/upgradepath3

Drop upgrade hook autodetection