CHROMIUM: add link to our blocker documentation

As people migrate files between packages, it seems the use of
blockers is missed.  Add some links to the error message.

BUG=None
TEST=emerging a package w/blockers shows a nice error

Change-Id: I775d57b2a7298f9c3d6039dbb4c2922a5eb1d35d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/portage_tool/+/1808141
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed