Failed network element creation leads to the return of false error messages

There are multiple occasions of the wrong error usage within controller/northbound/server/networkelement.go in the addMne method (https://code.fbi.h-da.de/danet/gosdn/-/blob/b406addb4a7965c7f7a0eac769ca72d7f9115802/controller/northbound/server/networkElement.go#L571)

Description

One example of a typo:

if err != nil {
	if pluginRmErr := plugin.Remove(); err != nil {
		return uuid.Nil, pluginRmErr
	}
	return uuid.Nil, err
}

which should be:

if err != nil {
	if pluginRmErr := plugin.Remove(); pluginRmerr != nil {
		return uuid.Nil, pluginRmErr
	}
	return uuid.Nil, err
}

Expected Behavior

The correct error should be returned

Actual Behavior

Wrong errors are returned and therefore sometimes a nil error is returned.

Possible Fix

Typos like the one I've mentioned above should be addressed. But in general I think a better way would be to do the clean up of a plugin within a defer function.

Steps to Reproduce

Context

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