Document how to distinguish exceptions from JSON, or encode exceptions as JSON

[davidsarah]

darrob: hello again. i have a python program that runs tahoe repair operations using urllib and parses tahoe's json responses. this works very well except for one thing: if a file is gone and i attempt a repair, the resulting NotEnoughSharesError? is not presented as json data but plaintext (a check without repair responds in json, however).

darrob: am i missing something on tahoe's side that i can do about it? if not, are there other error messages like this that i should be aware of before i adapt the program?

We need to document how to distinguish exceptions (I *think* valid JSON always starts with a "{" or "[" character).

An alternative would be to encode the exception as JSON (i.e. something like {"exception": "..."}), although that would make it more difficult to read. Since these APIs are mainly for programmatic use, that might be acceptible.