If you specify a directory incorrectly to roll_dep then it may not be
obvious why it cannot be found. For instance, "roll-dep tools\gyp" vs.
"roll-dep src\tools\gyp". This change prints the full directory.

This change also clarifies what is meant by a clean tree to make that
error message easier to understand.

R=maruel@chromium.org

Review URL: https://codereview.chromium.org/1507623003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@297875 0039d316-1c4b-4281-b951-d872f2087c98