Add additional info/warning/error popups or labels

[mschoema]

Add different info/warning/error popups or labels to the plugin.

Example use-cases:

• Send an error if the project name is empty
• Send an error if the query failed
• Send a warning if the query did not contain any temporal values

[mschoema]

Additionally, as a follow-up to a recent fix, the plugin could mention some information about the last completed query.
For example: Query returned X rows and generated Y layers (names)
This would also be shown if no rows were returned, otherwise we don't know if the query was successful or not since nothing happens.
Such a message could go to the right of the 'Refresh Layer' button.