SEP6 clarify error messaging

[msfeldstein]

For /withdraw and /deposit, we need to indicate that the error message is a string to be displayed to users, and also have a language option for localization.

Fixes #402

[morleyzhi]

Looks good, though I think we should also add documentation for a new error_metadata (or a different name that serves the same purpose.)

[msfeldstein]

What do you expect error_metadata to be used for?

[tomquisel]

Thanks! I'm not so pumped about the complexity of the extra lang parameter, but I don't know how else to do it. I'd also bring up that the how response field for /deposit should be internationalized based on lang

[morleyzhi]

@msfeldstein That's so the anchor can provide more structured data about the error, like a list of invalid inputs, for example.

[msfeldstein]

We can do a follow up for that if we need, im not sure how it would work since we would need to either define a set of available values, or clients and anchors would need to work together to come to consensus among themselves about what fields are and mean.