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/04-control-flow.md

@@ -231,6 +231,11 @@ Two variables are set when the trap fires: **`ER#`** is the error code and **`EL
 the line it happened on. `ERR(ER#)` gives the message text. On a C128 these are called
 `ER` and `EL` with no suffix; this dialect has no bare variable names.
 
+**[Chapter 15](15-error-codes.md) is the complete list** of what `ER#` can hold and
+what each code means. The short version is that the interpreter's own codes are 512 to
+519 and are the only numbers worth comparing against; two codes render as the same
+`ERR()` text, so compare the number rather than the text.
+
 `RESUME` comes in three forms:
 
 | Form | Where it goes |