Declare a SubtitleFormatParseError exception, its ancestry might not be final yet. (I'm thinking it needs a file format error class as base, which in turn inherits from agi::InvalidInput.) Updates #1213 in preparation for the actual patch.

Originally committed to SVN as r4556.

aegisub/src/subtitle_format.h

@@ -47,6 +47,8 @@
 #include <wx/string.h>
 #endif
 
+#include <libaegisub/exception.h>
+
 
 //////////////
 // Prototypes
@@ -161,3 +163,5 @@ public:
 };
 
 
+DEFINE_SIMPLE_EXCEPTION(SubtitleFormatParseError, agi::InvalidInputException, "subtitle_io/parse/generic")
+