WIP: GError: Adding informatin about error set point

g_set_error was converted to macro to include information about a place where the error is set.

Relevant discussion: #14 (closed) #1189

Right now, any error from GError is useless. The developer must manually add information where an error is set. There are a lot of old code that just calls g_set_error somewhere, somehow and we can only see the fact. I would like to embed this information into the error message. This is not for users and therefore we don't need to worry about the format. This is just a draft to start the conversation.