mb_regress.sh gripes

Hi all,

A few gripes about mb_regress.sh:
 1. No exit code is specified, so even if there are differences
between results/ and expected/ the script will still return 0.

 2. The 'dropdb' command is used to wipe out the "utf8" database
before the run. This generates an error message like:
  dropdb: database removal failed: ERROR:  database "utf8" does not exist

the first time you run the script. IMO it would be less startling to
just print a NOTICE here.

 3. No error checking for whether createdb succeeds.

The attached patch fixes these problems.

Josh