runtime+error: rename Too{Few,Many}Args => ...Varargs to reflect usage.
runtime+error: remove r5js.PrimitiveProcedureError; doesn't reflect R5RS.
runtime+error: remove vague r5js.GeneralSyntaxError.
runtime: use UnimplementedOptionError for bad environment specifiers.
test: improve test coverage of expected errors.
test: better coverage of immutable errors.