Show WARNINGS raised during migration to user

[leendertvanwolfswinkel]

In threedi-schema, a mechanism will be implemented to pass warnings to tools that run migrations nens/threedi-schema#112

• [3Di Results Analysis] Processing algorithm: add to existing logging framework using feedback.pushWarning()
• [3Di Models & Simulations a.k.a. threedi-api-qgis-client]
  • At the moment, migration is done immediately after download, but I think we can remove this functionality (there is already a ticket for this)
  • Upload wizard contains option to Check schematisation, which requires migration if schema is outdated
• [3Di Schematisation Editor] Migration is performed upon loading if schema is outdated

Probably best to implement this generically together with progress bar in threedi-mi-utils.

[margrietpalm]

@leendertvanwolfswinkel for the results analysis plugin I now have added the warnings as shown below. This is fine with one warning, but with many it may become to cluttered.

I can limit the number of times warnings are printed, and I can even do that by warning type. Is that something we want?

Not sure by the way if this will be an issue if all warnings are already nicely grouped like this.