[Browser][CoreCLR] ErrnoError spam in corerun

During startup for even a simple app like Hello World we generate dozens if not hundreds of ErrnoErrors that make it difficult to diagnose actual failures - you have to disable 'caught exceptions' in the debugger until exception reaches Just The Right Point, then turn them back on. If an exception actually happens during this startup phase you'll miss it.

These exceptions have a few sources:

`createDefaultDirectories` calls `mkdir`
`parse_args` calls `pal::get_absolute_path` calls `realpath`
`external_assembly_probe` calls `pal::try_map_file_readonly` calls `open`

If we were to migrate to emscripten's WasmFS these ErrnoErrors would go away, but it's not currently possible to do so (it might be possible after the emscripten bump, but it's unclear whether we want to do it).

cc @pavelsavara