add support for better error messages

This includes extended error messages which show the original format
string. It works for 'char' type with wchar_t returning only the
message.

The thrown exception class now has a format() method to retrieve the
pointer to the original format.

format.cc

@@ -409,12 +409,14 @@ void fmt::BasicFormatter<Char>::CheckSign(const Char *&s, const Arg &arg) {
 
 template <typename Char>
 void fmt::BasicFormatter<Char>::DoFormat() {
-  const Char *start = format_;
+  const Char *start = format_; // capture the format string before it is reset
+  const Char *original = format_;
   format_ = 0;
   next_arg_index_ = 0;
   const Char *s = start;
   typedef internal::Array<Char, BasicWriter<Char>::INLINE_BUFFER_SIZE> Buffer;
   BasicWriter<Char> &writer = *writer_;
+  try {
     while (*s) {
       Char c = *s++;
       if (c != '{' && c != '}') continue;
@@ -644,6 +646,11 @@ void fmt::BasicFormatter<Char>::DoFormat() {
         break;
       }
     }  
+  } catch (const FormatError &e) {
+    // rethrow FormatError with the format string pointed to by start
+    throw BasicFormatError<Char>(e.what(), original);
+  }
+  
   writer.buffer_.append(start, s);
 }
 
@@ -679,6 +686,17 @@ template void fmt::BasicFormatter<char>::CheckSign(
 
 template void fmt::BasicFormatter<char>::DoFormat();
 
+const char* FormatErrorMessage(const std::string &message, const char* format) {
+    return fmt::c_str(fmt::Format("error: {} while parsing {}") << message << format);
+}
+
+template<> fmt::BasicFormatError<char>::BasicFormatError(const std::string &message, const char *format)
+    : std::runtime_error(FormatErrorMessage(message, format)), format_(format) {}
+
+template<> fmt::BasicFormatError<char>::~BasicFormatError() {
+    std::runtime_error::~runtime_error();
+}
+
 // Explicit instantiations for wchar_t.
 
 template void fmt::BasicWriter<wchar_t>::FormatDouble<double>(
@@ -711,3 +729,10 @@ template void fmt::BasicFormatter<wchar_t>::CheckSign(
     const wchar_t *&s, const Arg &arg);
 
 template void fmt::BasicFormatter<wchar_t>::DoFormat();
+
+template<> fmt::BasicFormatError<wchar_t>::BasicFormatError(const std::string &message, const wchar_t *format)
+    : std::runtime_error(message), format_(format){}
+
+template<> fmt::BasicFormatError<wchar_t>::~BasicFormatError() {
+    std::runtime_error::~runtime_error();
+}

format.h

@@ -292,6 +292,16 @@ class FormatError : public std::runtime_error {
   : std::runtime_error(message) {}
 };
 
+template <typename Char>
+class BasicFormatError : public std::runtime_error {
+private:
+    std::basic_string<Char> format_;
+public:
+    explicit BasicFormatError(const std::string &message, const Char *format);
+    virtual ~BasicFormatError() throw();    
+    const Char *format() const { return format_.c_str(); }
+};
+
 enum Alignment {
   ALIGN_DEFAULT, ALIGN_LEFT, ALIGN_RIGHT, ALIGN_CENTER, ALIGN_NUMERIC
 };