"pub cache repair" prints a more detailed summary.

Now instead of just saying how many packages failed to be
reinstalled/reactivated, "pub cache repair" will print a list of those
packages.

This also fixes an issue where "pub cache repair" would incorrectly
return a 0 exit code when packages failed to reactivate.

Closes #1301

R=rnystrom@google.com

Review URL: https://codereview.chromium.org//1239623005 .