Create an Error object to represent GL errors.

BUG=angle:520

Change-Id: I1a54e6f308b5b0f310c725a7771af737a77ecd42
Reviewed-on: https://chromium-review.googlesource.com/209619
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/common/angleutils.h b/src/common/angleutils.h
index 7701a42..95f1b60 100644
--- a/src/common/angleutils.h
+++ b/src/common/angleutils.h
@@ -16,6 +16,7 @@
 #include <string>
 #include <set>
 #include <sstream>
+#include <cstdarg>
 
 // A macro to disallow the copy constructor and operator= functions
 // This must be used in the private: declarations for a class
@@ -131,6 +132,9 @@
     return strstr.str();
 }
 
+std::string FormatString(const std::string &fmt, va_list vararg);
+std::string FormatString(const std::string &fmt, ...);
+
 #if defined(_MSC_VER)
 #define snprintf _snprintf
 #endif