Sanity checks on RGI IDs

[fmaussion]

When users are making small mistakes on their IDs (such as forgetting a 0 or similar, it certainly happened to many, but not @Morlocke), it would be good if OGGM could fail with a better error message than "file is not here". User friendliness and all...

[Morlocke]

Nope, definitely didn't cause me to nearly lose my sanity today, because I am not a complete tool. No sirree bob.