Couple small error handling fixups

OpenAL32/alExtension.c

@@ -56,10 +56,10 @@ AL_API ALboolean AL_APIENTRY alIsExtensionPresent(const ALchar *extName)
     Context = GetContextRef();
     if(!Context) return AL_FALSE;
 
-    if(!extName)
-        alSetError(Context, AL_INVALID_VALUE);
-    else
+    al_try
     {
+        CHECK_VALUE(Context, extName);
+
         len = strlen(extName);
         ptr = Context->ExtensionList;
         while(ptr && *ptr)
@@ -78,6 +78,7 @@ AL_API ALboolean AL_APIENTRY alIsExtensionPresent(const ALchar *extName)
             }
         }
     }
+    al_endtry;
 
     ALCcontext_DecRef(Context);
     return ret;

OpenAL32/alSource.c

@@ -699,8 +699,7 @@ AL_API void AL_APIENTRY alSource3i(ALuint source, ALenum param, ALint value1, AL
                 break;
 
             default:
-                alSetError(Context, AL_INVALID_ENUM);
-                break;
+                al_throwerr(Context, AL_INVALID_ENUM);
         }
     }
     al_endtry;