Give better error message in weatherbench/metrics when climatology doesn't contain forecast keys. Previously, the KeyError raised by xr would indicate that "some_var_mean" was not found, but that "_mean" variable was likely just a "let's do a final attempt" at finding variables, and this confuses the user.

Give better error message in weatherbench/metrics when climatology doesn't contain forecast keys. Previously, the KeyError raised by xr would indicate that "some_var_mean" was not found, but that "_mean" variable was likely just a "let's do a final attempt" at finding variables, and this confuses the user. #421