Don't crash on error codes passed to `--explain` which exceed our internal limit of 9999
removed panic in case where we do `--explain > 9999` and added check for it
now error looks like this instead of ICE
```
$ rustc.exe --explain E10000
error: E10000 is not a valid error code
```
fixes#140647
r? `@fmease`
Guillaume Gomez
7d372aec2f