Feature proposal: replace "some projects have trouble loading" message with diagnostics

The template doesn't apply here, so I'm purposefully ignoring it.

This is a feature proposal to replace the "some projects have trouble loading" message (in WarningMessageObserver.ts) with diagnostics instead.

Here's my reasoning behind this proposal:
* The existing message gives very little context to what's wrong. "Show Output" just dumps you into the (quite detailed) O# log, with no indication of what to look for or how to fix it. In fact, until I looked through the source code this afternoon, I had no clue I was supposed to look at the unprefixed lines, _not_ the `[warn]` or `[fail]` lines.
* The message is distracting and pops up every 1500ms as applicable. If I know these errors are harmless, I don't want to keep closing them as the solution loads. (The extension setting to turn these messages off is something I'd prefer to not set, because it may be that errors in other solutions do need to be surfaced.)
* Visual Studio already shows MSBuild errors in its Problem panel (or whatever its VS Code equivalent is called). This makes the MSBuild error scenario consistent between the two.
* Diagnostics allows for easy discoverability of the offending error and filename/column (though admittedly, the filename is usually a red herring in the VS installation and less than useful).

Alternatively, the info message can be kept, but instead of asking them to sift through O# logs, point them to the Problems View instead.

I'm happy to take on this work if the team agrees with the proposal. Here's an example of what it could look like: