tool: improve config error messaging

- make sure that errors for specific options in config files identify
  the file, line number and shows the error about the correct option

- improve some error message wording

- add warning for leading single quote of arguments in config files
  (verified in test 1712)

- adjust test error outputs accordingly

test1712 introduces mode=warn

Use the mode="warn" attribute if the output curl warning output, as it
then makes the check without newlines and the prefix to better handle
that the lines may wrap at different points depending on the lengths of
the lines and terminal width.

Fixes #20598
Closes #20666

docs/tests/FILEFORMAT.md

@@ -712,12 +712,16 @@ server is used), if `nonewline` is set, we cut off the trailing newline of
 this given data before comparing with the one actually sent by the client The
 `<strip>` and `<strippart>` rules are applied before comparisons are made.
 
-### `<stderr [mode="text"] [nonewline="yes"] [crlf="yes|headers"]>`
+### `<stderr [mode="text/warn"] [nonewline="yes"] [crlf="yes|headers"]>`
 This verifies that this data was passed to stderr.
 
 Use the mode="text" attribute if the output is in text mode on platforms that
 have a text/binary difference.
 
+Use the mode="warn" attribute for curl warning output, as it then makes the
+check without newlines and the prefix to better handle that the lines may wrap
+at different points depending on the lengths of the lines and terminal width.
+
 `crlf=yes` forces the newlines to become CRLF even if not written so in the
 test.