buildman: Use an object to hold error lines

buildman: Use an object to hold error lines

At present the string for each error line is created in _CalcErrorDelta()
and used to create the summary output. This is inflexible since all the
information (error/warning character, error line, list of boards with that
error line) is munged together in a string.

Create an object to hold this information and only convert it to a string
when printing the actual output.

Signed-off-by: Simon Glass <sjg@chromium.org>