A more precise description of no-…-description conditions

[38a938c2]

Right now implementation says that a name being not present in a lexical environment is a program-error (which seems a bit off, as variable-description is probably the recommended way of checking if a variable exists in an environment, and program-error is expected to be used for syntax-level errors). At the same time, specification neither says that the return value is nil in case the variable doesn't exist, nor that the condition signalled is a serious condition. This could lead to divergence between implementation-specific Trucler protocol support libraries.

Personally, I think returning NIL after signalling a non-serious condition would make the most sense.