upgrade-master skips database creation.

[verm]

Under certain conditions upgrade-master will skip creating the initial database.

Here is what I did to hit this:

1. buildbot create-master ./buildmaster
2. Replace master.cfg with new config file.
3. Update c['db'] with proper mysql connector string.
4. Start buildbot
5. Receive error about requiring upgrade-master
6. Run upgrade-master, this passes w/out error.
7. Re-start buildbot which produces the same error as in #5
8. Observe that mysql database is infact empty with no tables.

In order to get this working, I had to run buildbot create-master --db='[connector string]' This gave me a working database to start from.

The upgrade-master tool needs to detect this condition and suggest starting from scratch. At the very least it should confirm that the required tables have been created w/out errors and produce a suitable error message in case of failure.

[PiotrSikora]

Information about proper steps (would upgrade-master --db='..' work or does it work only with create-master?) should be added to the exception message located in master/buildbot/master.py at line 997.

[Dustin J. Mitchell]

Better error messaes and docs for MySQL setup

Fixes #989

Changeset: cdb6915d49e83a944a3d7572c20fc719a28882ec