Return 413 when image_size_cap exceeded

HTTP Error 413 (Request entity too large) may be the most appropriate
response when a request to upload an image whose size exceeds
image_size_cap is made.

Addresses bug 1158711.

Change-Id: I4abcb3a8b57c080b5a1b512f9694b2d75a4e5e9d
Merge "Return 413 when image_size_cap exceeded"