Merge pull request #2530 from lambdageek/monoerror-mono_string_new

Merge pull request #2530 from lambdageek/monoerror-mono_string_new

[runtime] Use MonoError for mono_string_new

We are refraining from changing runtime callers of `mono_string_new` to `mono_string_new_checked` in one commit. String creation is used in several hundred places and threading error handling through all the callers is very intrusive. Additionally, AFAIK the only error condition from mono_string_new is out-of-memory. In a lot of cases there's no way to meaningfully recover.

We should revisit individual callers when it makes sense, but just mechanically updating everywhere doesn't get us a lot of benefit.