Better error/message handling

[owzim]

... when installing/uninstalling.
At the moment this is not optimal. Further ideas to be added here.

[pieterbeulque]

Yeah, it's not ideal but in my opinion the overall ProcessWire notice handling with $this->error() and $this->message() isn't that good. We should stay with the platform standards anyway.

We could try to improve the content of the messages but I think we're being pretty clear showing a success message and all added/edited/removed fields.