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/README.md

@@ -31,6 +31,7 @@ embedding it, debugging it or changing it.
 | **[12. Function reference](12-function-reference.md)** | Every function, alphabetically |
 | **[13. Differences from BASIC 7.0](13-differences.md)** | What a C128 programmer needs to know |
 | **[14. Architecture](14-architecture.md)** | How the interpreter is put together, how to debug it, how to change it |
+| **[15. Error codes](15-error-codes.md)** | Appendix: every value `ER#` can hold and every error line the interpreter prints |
 
 ## The shortest possible start