Design error handling

[seanmiddleditch]

Currently, a malformed format spec, out of range argument, or other runtime problem just silently terminates formatting and returns.

Design a "proper" error handling solution.

Exceptions would not fit the target userbase. Possible options include:

• terminate (with macro to replace call with custom termination handler?)
• Return error code instead of, or in addition to, the char*
• Add overloads that take an error code output parameter
• Write detailed error string into output
• Some combination of the above