Include output of failed commands in exception messages.

Without this the output is logged, but there is not necessarily
a log present.
Increase command length to 2000 characters.

255 chars is insufficient for more involved setups.

On modern Unices the limit is about 256k chars I think.

This change will not affect existing installations.
Those using sqlite will need to delete and recreate the
database. Users of other database engines can perform
equivalent changes manually on their databases.