Make Hover error messages much better

[tomcl]

We now have no bugs in the hover error message mechanism, and ability to write messages as markdown with working links. So we need to add proper errors.

BUT - to do this sensibly we need to autogenerate:

• HTML pages for opcode full documentation
• Quick ref operand syntax documentation
• Hover message markdown strings

We should publish HTML on an external site (I guess Github pages for an external public access Visual2 project) and use these both for documentation and links in hover messages.

We should create syntax doc automatically from op-codes (since many opcodes have same syntax).

I'll do this over Summer if no other takers.

[tomcl]

This is mostly done now. Also we have Tippy hover tooltips, separately from monaco, which are more flexible