Document every error code a script can see, as chapter 15

ER# held numbers nothing explained. The appendix lists the four error classes
the interpreter prints, the eight codes it owns and what raises each, and the
codes that reach ER# from errno, libakerror and libakgl underneath it.

The table is not asserted. A program in the chapter trips seven of the eight and
prints what it got, and docs_examples byte-compares the result -- so the numbers
are checked rather than claimed. The eighth, 516, is not usefully trappable and
the chapter says why: entering a handler takes a scope, and the pool being empty
is what raised it.

Two things worth a reader's attention came out of writing it. Two codes register
the same ERR() text, so a program must compare the number and print the text.
And VAL reports libakerror's Value Error rather than the interpreter's 517,
which makes that number the platform's rather than ours -- filed as section 6
item 21, not fixed here, because deciding which libakstdlib failures to
translate is a boundary question and ENOENT out of DOPEN is the counter-case.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/12-function-reference.md

@@ -54,4 +54,5 @@ this dialect has no variable name without a type suffix:
 
 `ER#` carries this interpreter's error code, not a Commodore error number. There is no
 correspondence to reproduce — the errors raised here are not the errors a 1985 ROM
-raised — so print `ERR(ER#)` rather than the number.
+raised — so print `ERR(ER#)` rather than the number. **[Chapter 15](15-error-codes.md)
+lists every value it can hold.**