SEP0006: Make it clear that error responses should be strings

[morleyzhi]

/deposit and /withdraw endpoints sometimes return errors. The examples indicate that they should return a property error that's a string, but that's not made clear in the spec's language.

So my suggestion is:

• Canonize the error response property, required for errors, and require that it be a string. It should be human-readable and user-friendly.
• Add an optional error_metadata property that must be an object, and can contain more information about the error.