improved error messages - missing psql/mysql binaries

[JohnMurray]

See issue #36 for context.

Rather than throwing an exception all the way up the stack we should

1. Catch the issue earlier and provide a better output message (possibly depending on the -v option how much of it we show vs a normal failure)
2. Provide a specific error message / check when the binary cannot be found (either psql or mysql)

It might also make sense to provide a configuration option that allows you to provide a path to the specific pgsql or mysql binary that you would like to use, with the default value (if no config value provided) being whatever the script can locate on the current path.