Bug 1196391, part 2 - Add JSAPI constant for the max number of error arguments. r=Waldo

This will allow users of the API to do checking of values they pass in.

js/src/jsapi.h

@@ -4478,6 +4478,10 @@ JS_GetLocaleCallbacks(JSRuntime* rt);
  * Error reporting.
  */
 
+namespace JS {
+const uint16_t MaxNumErrorArguments = 10;
+};
+
 /*
  * Report an exception represented by the sprintf-like conversion of format
  * and its arguments.  This exception message string is passed to a pre-set

js/src/jscntxt.cpp

@@ -582,9 +582,9 @@ js::ExpandErrorArgumentsVA(ExclusiveContext* cx, JSErrorCallback callback,
         reportp->exnType = efs->exnType;
 
         size_t totalArgsLength = 0;
-        size_t argLengths[10]; /* only {0} thru {9} supported */
+        size_t argLengths[JS::MaxNumErrorArguments]; /* only {0} thru {9} supported */
         argCount = efs->argCount;
-        MOZ_ASSERT(argCount <= 10);
+        MOZ_ASSERT(argCount <= JS::MaxNumErrorArguments);
         if (argCount > 0) {
             /*
              * Gather the arguments into an array, and accumulate