Include the topic in some common parser errors

[kirsle]

This includes the current topic in some common errors from the parser. Examples:

Weird single-character line 'i' found (in topic random) at tmp/test.rive line 7
Unknown command '$' (in topic random) at tmp/test.rive line 8

The errors affected:

• Syntax errors.
• Weird single-character lines.
• Unknown command symbol.

Fixes #140