Upgrade reads and modifies error messages

[MikaelMayer]

What change in documentation do you suggest?

Currently, reads errors look like this:

Error: insufficient reads clause to invoke function

which is very hard to decipher, has no actionable item, does not seem linked to verification.
I think an error message like this would help disambiguate what's going on:

Inside methods
Error: could not prove that the function's reads consists only of fresh objects or object/fields that the enclosing method declares to read

Inside functions
Error: could not prove that the function's reads consists only of object/fields that the enclosing function declares to read