handle modifications on lines with an error

when processing a document change, if only a single line is modified,
the line number of the line is checked to see if it has an error and if
it does, the error will be removed from the errors list if the cursor
is within the error or when the cursor and error is at the end of the
line, or the error column is adjusted if the modification occurs before
the error (no action is take if the cursor is after the error)

to support the adjusting of the error column, new functions were added
to the error list and error item classes