upgrade Aspen
use new JSON error format provided by Aspen

fixes #1850
pass back the invalid username even if it has invalid characters
Adjust to negotiated error simplate.
test that the JSON endpoint returns a proper error message
update username tests to expect the real JSON error object
okay then, I'll use a hack :-/
minor changes
Merge pull request #2047 from gittip/1850-username-change-error

[1850] get the username change errors working again