ircd::json: Add why() to specifically return err str from valid().

include/ircd/json/json.h

@@ -138,7 +138,10 @@ ircd::json::print(const mutable_buffer &buf,
 	if(unlikely(!valid(sv, std::nothrow))) //note: false alarm when T=json::member
 		throw print_error
 		{
-			"print (%zu): %s", sv.size(), sv
+			"print %zu bytes: %s: %s",
+			 sv.size(),
+			 why(sv),
+			 sv
 		};
 
 	buf[sv.size()] = '\0';
@@ -161,13 +164,19 @@ ircd::json::strung::strung(T&&... t)
 		if(unlikely(size(sv) != size(out)))
 			throw print_error
 			{
-				"stringified:%zu != serialized:%zu: %s", size(sv), size(out), sv
+				"stringified:%zu != serialized:%zu: %s",
+				size(sv),
+				size(out),
+				sv
 			};
 
 		if(unlikely(!valid(sv, std::nothrow))) //note: false alarm when T=json::member
 			throw print_error
 			{
-				"strung (%zu): %s", size(sv), sv
+				"strung %zu bytes: %s: %s",
+				size(sv),
+				why(sv),
+				sv
 			};
 
 		return sv;

include/ircd/json/util.h

@@ -37,6 +37,7 @@ namespace ircd::json
 	// Validate JSON - checks if valid JSON (not canonical).
 	bool valid(const string_view &, std::nothrow_t) noexcept;
 	void valid(const string_view &);
+	std::string why(const string_view &);
 }
 
 inline std::ostream &

ircd/json.cc

@@ -2175,6 +2175,18 @@ ircd::json::operator==(const value &a, const value &b)
 // json.h
 //
 
+std::string
+ircd::json::why(const string_view &s)
+try
+{
+	valid(s);
+	return {};
+}
+catch(const expectation_failure &e)
+{
+	return e.what();
+}
+
 bool
 ircd::json::valid(const string_view &s,
                   std::nothrow_t)