[flang][driver] Update error messages (nfc)

As per Flang's coding guidelines
(flang/docs/C++style.md#error-messages):
```
Messages should start with a capital letter.
```

This patch updates error messages in the driver (new and old) so that
they conform with the guideline above.

This change was suggested in one of the recent reviews:
https://reviews.llvm.org/D93712. It felt like this deserved a dedicated
patch, so sending it separately.