pgdump plugin does not handle missing commands gracefully

[Andrew Garner]

Like most holland plugins, pg_dump runs commands through subprocess rather than via a shell. If a command is not found, subprocess raises an OSError with errno = ENOENT. This results in a messy and confusing failure message.

These invocations should be wrapped around a try/except to detect these cases and fail cleanly.

This particular case is common with Postgres on RHEL/Cent as newer versions installed from yum.postgresql.org are probably going to put their tools in /usr/pgsql-{version}/bin/ and not necessarily be in the path. holland's default path doesn't pick these up automatically.

[Andrew Garner (muzazzi)]

Improved the error handling for these cases so it's a little less ugly when these fail.

Featured branch here for inclusion in holland 1.0.12: https://github.com/holland-backup/holland/tree/feature/lp_1206202_handle_pgdump_missing_cmd