Off-by-one errors in assembler error messages

[jamesyoungman]

It looks like we have some off-by-one errors in the assembler error messages.

This is caused by inconsistency in whether we number lines and columns from 0 or 1. I think somewhere we're adding 1 twice.

[jamesyoungman]

The line numbers are fixed now. But there is a problem with the column numbering. This is because we assume one char occupies one column. This is not true for glyphs like circle and square, which don't advance the column (we implement these as combining chars).

This complexity makes this a less good fit for a beginner, so removing the "good first issue" tag.