Remove redundant error messages on configuration loading

[damyan]

When misconfiguring a plugin, the error messages are stacked to something like:
FATAL main: Failed to start server: invalid configuration: invalid configuration: invalid configuration: exactly one argument must be passed to the plugin, got 2

Set the error message in the low-level proc only and pass it to the output.
Further, change the implementation so the name of the misconfigured plugin is part of the error message.

Note: Shall be implemented for all plugins.