improve the error message on section mismatch

Author: gasche

bin/test/errors/parsing-errors.t

@@ -98,7 +98,7 @@ Mismatch between section-start and section-end:
   $ mustache foo.json $PROBLEM
   Template parse error:
   File "foo-bar.mustache", lines 1-2, characters 23-0:
-  Mismatched section 'foo' with 'bar'.
+  Section mismatch: {{#foo}} is closed by {{/bar}}.
   [3]
 
   $ PROBLEM=foo-not-closed.mustache
@@ -113,5 +113,5 @@ Mismatch between section-start and section-end:
   $ mustache foo.json $PROBLEM
   Template parse error:
   File "wrong-nesting.mustache", lines 1-2, characters 41-0:
-  Mismatched section 'foo' with 'bar'.
+  Section mismatch: {{#foo}} is closed by {{/bar}}.
   [3]

lib/mustache.ml

@@ -219,7 +219,7 @@ let pp_template_parse_error ppf ({ loc; kind; } : template_parse_error) =
   | Parsing ->
     p ppf "syntax error"
   | Mismatched_section { start_name; end_name } ->
-    p ppf "Mismatched section '%a' with '%a'"
+    p ppf "Section mismatch: {{#%a}} is closed by {{/%a}}"
       pp_dotted_name start_name
       pp_dotted_name end_name
   end;