Clearer error for 'conda search' with invalid regex

Closes gh-706

This prints a brief error message saying that the pattern was invalid,
rather than showing the full traceback.
Merge pull request #724 from takluyver/search-regex-err

Clearer error for 'conda search' with invalid regex
conda clean: Don't exit when there are no tarballs

There might be additional clean actions to perform.

Fixes #753.
Merge branch 'master' of github.com:conda/conda
Restore the ConnectionError handler

Fixes #755
Always fail with connection error in download

It can't do anything if the resp is not defined, and the connection error is
raised by get, not raise_for_status.
Fix conda update to correctly not install packages that are already up-to-date

It was checking the build string, not the build number, so it was thinking
that things like np18py26_0 was newer than np18py27_0.
formating