Improve error handling

[petertiedemann]

Title

When docfx fails, it is very hard to figure out even what project was the cause and a single failing project will immediately abort the whole process.

Functional impact

Makes it hard to debug problems.

Minimal repro steps

Anything that causes docfx to fail to process

Expected result

Docfx to catch and log exceptions on problematic projects, indicating clearly what projects failed, but continue processing remaining projects.

Actual result

See the output from #1345 as an example.

[bitbonk]

I hade a similar issue when the Microsoft Build Tools for Visual Studio 2015 were not installed. There was nothing in the Exception and its long stacktrace that could have given me the hint that this is the cause.

[vwxyzh]

#1670

[vicancy]

#1738