Make all errors static

[Manishearth]

Over FFI, any allocated errors can't be easily debugged since we return std::string_view (&'static str). This is done to reduce allocations.

The ErrorMessage enum is an attempt to consolidate error messages, but we also have some errors constructed with format!(). Ideally, we should not have any of them.

Potentially we can make the ixdtf errors pure strings as well, they only have a single non-unit variant.