Rise error if no grib

[scyrock]

When running a routing process without a GRIB file present in the area, the system currently throws the following error from libweatherrouting: Error: 'NoneType' object has no attribute 'isochones'
This message is unclear to the user and doesn’t explicitly explain that the issue is due to the absence of a GRIB file.
To improve the user experience, the error handling should be adjusted to clearly indicate the cause of the problem (missing GRIB file). Possible approaches:

1. Before calling libweatherrouting, implement a check in gww.
2. Capture the specific error raised by libweatherrouting and replace or wrap it with a clearer message.