Raise detailed RuntimeError if initdb exits non-zero
Raise detailed RuntimeError if initdb exits non-zero

With this change an exception is always raised if initdb encounters an error.
Examples:

# RuntimeError: command not found: initdb
# RuntimeError: failed to spawn initdb: [Errno 13] Permission denied
# RuntimeError: initdb failed: 'initdb: unknown option -- H\nTry "initdb --help" for more information.\n'

tk0miya/testing.postgresql,lpsinger/testing.postgresql