Return a `next-error' capable buffer.
If AVOID-CURRENT is non-nil, treat the current buffer
as an absolute last resort only.
The function EXTRA-TEST-INCLUSIVE, if non-nil, is called in each buffer
that normally would not qualify. If it returns t, the buffer
in question is treated as usable.
The function EXTRA-TEST-EXCLUSIVE, if non-nil, is called in each buffer
that would normally be considered usable. If it returns nil, that buffer is rejected.