Improve toaster message on network connection

[Sebastiaan127001]

Is your feature request related to a problem? Please describe.
I'm always frustrated when I read a message but cannot determine what the problem is.

Describe the solution you'd like
I would like the toaster message to describe which source cannot be refreshed or which metric cannot be refreshed.

Additional context

[fniessink]

Quality-time tries to fetch measurements for all metrics in the report at once, so there's not a specific metric or source that can be mentioned here. Is there a way to improve the message to make that more clear?