The directory sites/default/files exists but is not writable and could not be made writable
RHEL and Fedora (among others) may have SELinux security enable.  This will mask whatever the regular Unix permissions are showing.  If this is enabled and there is not a "rw" in the context for the files/ directory it will not be writable no matter which group, user or unix permissions are granted.  To see these "hidden" settings:

ls -laZ sites/default

drwxr-sr-x. apache www unconfined_u:object_r:httpd_sys_content_t:s0 .
drwxr-xr-x. apache www unconfined_u:object_r:httpd_sys_content_t:s0 ..
-rwxr-sr-x. apache www unconfined_u:object_r:httpd_sys_content_t:s0         default.settings.php
drwxrwsr-x. apache www unconfined_u:object_r:httpd_sys_content_t:s0 files
-rwxr-sr-x. apache www unconfined_u:object_r:httpd_sys_rw_content_t:s0 settings.php


To change the settings on files/ so it is writable:

sudo chcon -R -t httpd_sys_rw_content_t files


When the install is done reset the context using the same command with the original "http_sys_content_t" context. 

fyi - This command will also be necessary to enable writes when installing themes under the "all" directory.