more tailored AQL error messages in case collection cannot be found
INCOMPATIBLE CHANGE: changed AQL user function namespace resolution operator from `:` to `::`

AQL user-defined functions were introduced in ArangoDB 1.3, and the namespace resolution
operator for them was the single colon (`:`). A function call looked like this:

    RETURN mygroup:myfunc()

The single colon caused an ambiguity in the AQL grammar, making it indistinguishable from
named attributes or the ternary operator in some cases, e.g.

    { mygroup:myfunc ? mygroup:myfunc }

The change of the namespace resolution operator from `:` to `::` fixes this ambiguity.

Existing user functions in the database will be automatically fixed when starting ArangoDB
1.4 with the `--upgrade` option. However, queries using user-defined functions need to be
adjusted on the client side to use the new operator.