Also change several error messages to refer to "items" rather than
"methods", since associated items that require resolution during type
checking are not always methods.
This allows more errors to be non-fatal, as per #1871.
I only went through and started changing span_fatal to span_err in
check.rs. There are probably more errors that could be made
non-fatal. So if you see derived type errors appearing from now on,
file a bug!
r=graydon
Closes#1871
