no "out of space" error for "save as"

[gue5t gue5t]

To reproduce:
On linux:
$ mkdir /tmp/tmpfs0b
$ sudo mount -t tmpfs -o size=1K,mode=777 tmpfs /tmp/tmpfs0b
$ dd if=/dev/zero of=/tmp/tmpfs0b/4k count=4096 bs=1
$ midori -p
Visit <http://www.ti.com/lit/ds/symlink/lmz14202ext.pdf> and click "save as" on the download dialog, placing in the /tmp/tmpfs0b directory. An out-of-space error message is shown.
Then save an image (right-click, "save image") to the /tmp/tmpfs0b directory. Notice that no out-of-space warning nor failure occurs, and the image silently fails to save.

We should ensure that the actual writes succeed, as well as performing the pre-emptive download-has-enough-space check when doing a regular save operation.