I had a failure while building/installing gnome and I am not sure what
the failure is.  Here is the first mention of trouble

/bin/sh /var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/work/gnome-user-docs-2.26.2/install-sh -d /var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome/help/user-guide/C/figures/
/usr/bin/install -c -m 644 C/figures/ask_pointer.png /var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome/help/user-guide/C/figures/ask_pointer.png
mkdir: cannot create directory `/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome': File exists
mkdir: cannot create directory `/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome/help/user-guide/C': File exists
make[3]: *** [install-doc-docs] Error 1
make[3]: *** Waiting for unfinished jobs....

It seems to be complaining that /usr/share/gnome exists and that
/usr/share/gnome/help/user-guide/C exists

But that doesn't seem like an error for an upgrade.

It then has 66 successful installs like this one (I guess these are the
unfinished jobs)

/usr/bin/install -c -m 644 C/figures/busy_pointer.png /var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome/help/user-guide/C/figures/busy_pointer.png

and finally here is the tail of the log

/usr/bin/install -c -m 644 C/figures/yelp_window.png /var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/image//usr/share/gnome/help/user-guide/C/figures/yelp_window.png
make[3]: Leaving directory `/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/work/gnome-user-docs-2.26.2/gnome2-user-guide'
make[2]: *** [install-data-am] Error 2
make[2]: Leaving directory `/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/work/gnome-user-docs-2.26.2/gnome2-user-guide'
make[1]: *** [install-am] Error 2
make[1]: Leaving directory `/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/work/gnome-user-docs-2.26.2/gnome2-user-guide'
make: *** [install-recursive] Error 1
 * 
 * ERROR: gnome-extra/gnome-user-docs-2.26.2 failed.
 * Call stack:
 *               ebuild.sh, line   49:  Called src_install
 *             environment, line 3016:  Called gnome2_src_install
 *             environment, line 2395:  Called die
 * The specific snippet of code:
 *           emake DESTDIR="${D}" "scrollkeeper_localstate_dir=${D}${sk_tmp_dir} " "$@" install || die "install failed";
 *  The die message:
 *   install failed
 * 
 * If you need support, post the topmost build error, and the call stack if relevant.
 * A complete build log is located at '/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/gnome-extra/gnome-user-docs-2.26.2/temp/environment'.
 * 

Finally attached is the entire build log