Yet more work on removing other/1 errors.

src/json.m:
src/json.json_lexer.m:
	Add specific errors for negations not followed by digits and bad
	comment starts.

	Fix a problem where a colon was not being placed after a context
	in some error messages.

tests/bad_comment.{json,exp}:
tests/bad_comment2.{json,exp}:
	Test the new error message for comment starts.

tests/zinc_soln_sep.exp:
	Test the new error message for negated numbers.

tests/*.exp:
	Fix error messages with missing colons.