Fix error logging in ValidateBlitFramebufferANGLE

Bundle an error message about whole buffer copies inside gl::Error
instead of delivering it to the Platform.

BUG=angleproject:1660

Change-Id: I13205b74529fbe40af05e7c1896c6315486e90e0
Reviewed-on: https://chromium-review.googlesource.com/437628
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
diff --git a/src/libANGLE/validationES2.cpp b/src/libANGLE/validationES2.cpp
index 13fcfe6..b107f68 100644
--- a/src/libANGLE/validationES2.cpp
+++ b/src/libANGLE/validationES2.cpp
@@ -1910,9 +1910,9 @@
                                   dstX0, dstY0, dstX1, dstY1))
                 {
                     // only whole-buffer copies are permitted
-                    ERR() << "Only whole-buffer depth and stencil blits are supported by this "
-                             "implementation.";
-                    context->handleError(Error(GL_INVALID_OPERATION));
+                    context->handleError(Error(GL_INVALID_OPERATION,
+                                               "Only whole-buffer depth and stencil blits are "
+                                               "supported by this extension."));
                     return false;
                 }