cros_workon: misc fixes to manifest re-generation
cros_workon: misc fixes to manifest re-generation

* Only run the regenerate once, after start
* Make regenerate preserve old projects from local manifest
* Make pkgname not found a non-fatal error
* Delete misc debug messages

	modified:   cros_workon

Review URL: http://codereview.chromium.org/3115001

bcwaldon/coreos-scripts,fivethreeo/scripts,zhang0137/scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,BugRoger/scripts,endocode/scripts,mjg59/scripts,glevand/coreos--scripts,mischief/scripts,crawford/scripts,endocode/scripts,nicescale/scripts,mischief/scripts,zhang0137/scripts,andrejro/coreos--scripts,mjg59/scripts,crawford/scripts,trnubo/scripts,mjg59/scripts,smilart/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,smilart/scripts,endocode/scripts,trnubo/scripts,crawford/scripts,smilart/scripts,zhang0137/scripts,coreos/scripts,vmware/coreos-scripts,marineam/coreos-scripts,gtank/scripts,nicescale/scripts,marineam/coreos-scripts,sigma/coreos-scripts,sigma/coreos-scripts,trnubo/scripts,coreos/scripts,gtank/scripts,marineam/coreos-scripts,sigma/coreos-scripts,fivethreeo/scripts,nicescale/scripts,mischief/scripts,gtank/scripts,glevand/coreos--scripts,BugRoger/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,vmware/coreos-scripts,BugRoger/scripts