json object for FileNotFound

When a nonexistant file is passed to a command and  --json-error-messages
is enabled, output a JSON object indicating the problem.

(But git ls-files --error-unmatch still displays errors about such files in
some situations.)

I don't like the duplication of the name of the command introduced by this,
but I can't see a great way around it. One way would be to pass the Command
instead.

When json is not enabled, the stderr is unchanged. This is necessary
because some commands like find have custom output. So dislaying
"find foo not found" would be wrong. So had to complicate things with
toplevelFileProblem having different output with and without json.

When not using --json-error-messages but still using --json, it displays
the error to stderr, but does display a json object without the error. It
does have an errorid though. Unsure how useful that behavior is.

Sponsored-by: Dartmouth College's Datalad project

doc/todo/api_for_telling_when_nonexistant_or_non_git_files_passed/comment_8_2550e1760dcfb90c9c2ca1ee145adcf1._comment

@@ -0,0 +1,19 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 8"""
+ date="2023-04-25T22:45:24Z"
+ content="""
+Ok, implemented the simple alternative. Here's how it looks:
+
+	joey@darkstar:~/tmp/xxx>git-annex add 'dne' --json --json-error-messages
+	{"command":"add","error-messages":["git-annex: dne not found"],"errorid":"FileNotFound","file":"dne","input":["dne"],"success":false}
+	add: 1 failed
+
+The errorid will remain stable. I can add those to other error messages
+now, on request BTW.
+
+Note that when git-annex relies on `git ls-files --error-unmatch` to
+complain about nonexistant or non-git files, the error messages from
+git will still be displayed to stderr, not this nice json. So
+datalad will need to keep its parser for that part.
+"""]]