Better error message for missing observatory location

[mdebony]

Users are confused by the error message from gammapy when observatory location is missing. We need to improve clarity.

[gabemery]

Could you clarify? is it an error when running BAccMod or later with gammapy?
If it is during baccmod we can probably catch the error and point to the instructions in the readme

[mdebony]

It's when running BAccMod. We are using it for the transformation back to camera coordinates.

[gabemery]

Ok then let's catch it where it may occur and give details on how to solve it