errors: prefix the output with "error: "

errors: prefix the output with "error: "

This improves readability. This is also what gcc does.

* NEWS: Document this change.
* src/complain.c (complain_at): Prefix all errors with "error: ".
(complain_at_indent, warn_at_indent): Do not prefix the context
information of errors, which are basically just indented errors.
* tests/conflicts.at, tests/glr-regression.at, tests/input.at,
tests/named-refs.at, tests/output.at, tests/push.at,
tests/regression.at, tests/skeletons.at: Apply this change.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>