Update inaccurate error string.

src/libANGLE/ErrorStrings.h

@@ -76,6 +76,7 @@ MSG kCubemapIncomplete = "Texture is not cubemap complete. All cubemaps faces mu
 MSG kCubemapInvalidDepth = "The cubemap depth must be a multiple of 6.";
 MSG kDataTypeNotAligned = "Data is not evenly divisible into the number of bytes needed to store in memory a datum indicated by type.";
 MSG kDefaultFramebuffer = "Default framebuffer is bound.";
+MSG kDefaultFramebufferAttachmentOnUserFBO = "Invalid attachment when a user framebuffer is bound.";
 MSG kDefaultFramebufferInvalidAttachment = "Invalid attachment when the default framebuffer is bound.";
 MSG kDefaultFramebufferInvalidDrawBuffer = "Only NONE or BACK are valid draw buffers for the default framebuffer";
 MSG kDefaultFramebufferTarget = "It is invalid to change default FBO's attachments";

src/libANGLE/validationES.cpp

@@ -3541,7 +3541,7 @@ bool ValidateDiscardFramebufferBase(const Context *context,
                     if (!defaultFramebuffer)
                     {
                         context->validationError(GL_INVALID_ENUM,
-                                                 kDefaultFramebufferInvalidAttachment);
+                                                 kDefaultFramebufferAttachmentOnUserFBO);
                         return false;
                     }
                     break;