Add support for try/catch

[BruceForstall]

1. Generate try/catch, with catch of System.Exception
2. Sometimes generate throw new System.Exception() if we're generating a statement within a try of a try/catch construct.
3. Generate try/catch/when(expression). In this case, don't generate throw because we don't know if the expression will be true.

Case (3) exposes lots of dotnet/runtime#114985, so add that assertion to the known errors list for dotnet/runtime.

[BruceForstall]

@jakobbotsch PTAL