Improve error messages for malformed array input strings.

Make the error messages issued by array_in() uniformly follow the style
	ERROR: malformed array literal: "actual input string"
	DETAIL: specific complaint here
and rewrite many of the specific complaints to be clearer.

The immediate motivation for doing this is a complaint from Josh Berkus
that json_to_record() produced an unintelligible error message when
dealing with an array item, because it tries to feed the JSON-format
array value to array_in().  Really it ought to be smart enough to
perform JSON-to-Postgres array conversion, but that's a future feature
not a bug fix.  In the meantime, this change is something we agreed
we could back-patch into 9.4, and it should help de-confuse things a bit.

src/pl/plperl/expected/plperl_array.out

@@ -65,9 +65,10 @@ ERROR:  number of array dimensions (7) exceeds the maximum allowed (6)
 LINE 1: select plperl_sum_array('{{{{{{{1,2},{3,4}},{{5,6},{7,8}}},{...
                                 ^
 select plperl_sum_array('{{{1,2,3}, {4,5,6,7}}, {{7,8,9}, {10, 11, 12}}}');
-ERROR:  multidimensional arrays must have array expressions with matching dimensions
+ERROR:  malformed array literal: "{{{1,2,3}, {4,5,6,7}}, {{7,8,9}, {10, 11, 12}}}"
 LINE 1: select plperl_sum_array('{{{1,2,3}, {4,5,6,7}}, {{7,8,9}, {1...
                                 ^
+DETAIL:  Multidimensional arrays must have sub-arrays with matching dimensions.
 CREATE OR REPLACE FUNCTION plperl_concat(TEXT[]) RETURNS TEXT AS $$
 	my $array_arg = shift;
 	my $result = "";