Improve error message for northing/easting not defined in English

At the moment, the warning message is not clear when submitting coordinates with easting written in French (O for W): 
```
parzer::parse_lon("53° 03′ 00' O")
[1] NaN
Warning message:
In base::.Call(...) : invalid characters, got: 53' 03' 00' o
```
