coredump,catalog: give better notice when a core file is truncated

coredump,catalog: give better notice when a core file is truncated

coredump had code to check if copy_bytes() hit the max_bytes limit,
and refuse further processing in that case.
But in 84ee0960443, the return convention for copy_bytes() was changed
from -EFBIG to 1 for the case when the limit is hit, so the condition
check in coredump couldn't ever trigger.
But it seems that *do* want to process such truncated cores [1].
So change the code to detect truncation properly, but instead of
returning an error, give a nice log entry.

[1] https://github.com/systemd/systemd/issues/3883#issuecomment-239106337

Should fix (or at least alleviate) #3883.