Not suitable for changing in a stable branch, in any case.

I finished off the branch as smcv/better-errors, and added another, 
smcv/even-better-errors. The former is relatively safe, the latter less so.

I can divide the better-errors branch up into 3 parts (TpBaseConnection, TpConnection, TpChannel Group) and get them reviewed separately, if one turns out to be controversial. Branches on the way in a moment (currently rebasing, since there were changes in the tests that are likely to conflict).

TpBaseConnection
================

When #TpBaseConnectionClass.start_connecting fails, any GError it yields is turned into a Connection_Status_Reason. I just added some more cases. This should be uncontroversial.

TpConnection
============

Old:
* Network_Error, "...ConnectionFailed" -> TP_ERROR_CONNECTION_FAILED
* Network_Error, "com.example.NoNetworkCoverage" -> TP_DBUS_ERROR_REMOTE_ERROR

New:
* Network_Error, "...ConnectionFailed" -> TP_ERROR_CONNECTION_FAILED
* Network_Error, "com.example.NoNetworkCoverage" -> TP_ERROR_NETWORK_ERROR

This also adds a function tp_connection_get_detailed_error(), which gets the ConnectionError arguments (if that signal was emitted) or a D-Bus error string chosen based on the invalidation reason (otherwise), and is necessary for Bug #21120.

TpChannel Group (in better-errors)
==================================

This changes the TpProxy::invalidated error if a D-Bus error string is seen in the "error" detail of MembersChangedDetailed. Previously, we used the Channel_Group_Change_Reason to construct an error in TP_ERRORS_REMOVED_FROM_GROUP; now, we use the "error", if recognised. If the "error" is not recognised, we still fall back to the Channel_Group_Change_Reason.

Old:
* Error, "...ConnectionFailed" -> domain=TP_ERRORS_REMOVED_FROM_GROUP, code=TP_CHANNEL_GROUP_CHANGE_REASON_ERROR
* Error, "com.example.NoNetworkCoverage" -> domain=TP_ERRORS_REMOVED_FROM_GROUP, code=TP_CHANNEL_GROUP_CHANGE_REASON_ERROR

New:
* Error, "...ConnectionFailed" -> TP_ERROR_CONNECTION_FAILED
* Error, "com.example.NoNetworkCoverage" -> domain=TP_ERRORS_REMOVED_FROM_GROUP, code=TP_CHANNEL_GROUP_CHANGE_REASON_ERROR

TpChannel Group (in even-better-errors)
=======================================

This additionally changes the behaviour of TpChannel's TpProxy::invalidated when there isn't an "error", or it isn't recognised. Instead of encoding the Channel_Group_Change_Reason into TP_ERRORS_REMOVED_FROM_GROUP, it's now encoded into TP_ERRORS, whenever possible, which makes TP_ERRORS_REMOVED_FROM_GROUP mostly obsolete.

As a special case, Channel_Group_Change_Reason_Error turns into the NotAvailable error code (I could change this if someone has a better idea).

Old:
* Kicked -> domain=TP_ERRORS_REMOVED_FROM_GROUP, code=TP_CHANNEL_GROUP_CHANGE_REASON_KICKED
* Error, "com.example.NoNetworkCoverage" -> domain=TP_ERRORS_REMOVED_FROM_GROUP, code=TP_CHANNEL_GROUP_CHANGE_REASON_ERROR

New:
* Kicked -> TP_ERROR_CHANNEL_KICKED
* Error, "com.example.NoNetworkCoverage" -> TP_ERROR_NOT_AVAILABLE

Sorry, I meant Bug #21200 rather than Bug #21120.

http://git.collabora.co.uk/?p=user/smcv/telepathy-glib-smcv.git;a=shortlog;h=refs/heads/better-errors

which is a prefix of:

http://git.collabora.co.uk/?p=user/smcv/telepathy-glib-smcv.git;a=shortlog;h=refs/heads/even-better-errors

It seems (from `git grep TP_ERROR`) that this won't impact Empathy at all, since Empathy does virtually nothing with errors. :'-(

better-errors looks good to me.
Didn't review even-better-errors yet.

I've merged most of better-errors; because even-better-errors hasn't been reviewed yet, I omitted the changes to TpChannel (I think if we're going to make changes to TpChannel's error behaviour, they should all land at once, so I'd like even-better-errors either accepted or rejected before merging the rest).

Looks good to me.

I've updated the branch with some text for NEWS. Merge is blocked by Bug #27967.

r+ from wjt for the NEWS, merging.

Fixed in git for 0.11.5