Improve internal error handling

[ToxicFrog]

At the moment, error handling is kind of a dog's breakfast of error() and assert() calls. Users can easily end up seeing a stack trace from vstruct internals.

This isn't terribly hard to fix:

• internal functions for internal, external, hard, and soft errors
• API functions should pcall the implementation
• turn error objects into appropriate throws or returns, including the internal stack trace if appropriate