Better error message when array is accessed in apply

[m-mohr]

Originates from Open-EO/openeo-r-client#53 (comment):

When array access is done in apply (not allowed), then GEE gives the following error message:

SERVER-ERROR: Server error: Dimension 'undefined' does not exist.

That is not ideal and should be improved nevertheless to return something like:

x is not an array