project.py: Consistently use the _error method to print error messages
project.py: Consistently use the _error method to print error messages

Use the _error method instead of directly calling `print`.

Also add a new _warn convenience method.

Change-Id: Ia332c14ef8d9d1fe2df128dbf36b5521802ccdf1

frogbywyplay/git-repo,mer-tools/git-repo,codetutil/git-repo,frogbywyplay/git-repo,gbraad/git-repo,nuclearmistake/repo,FuangCao/git-repo,aosp-mirror/tools_repo,gbraad/git-repo,gabbayo/git-repo,FuangCao/repo,GerritCodeReview/git-repo,dodocat/git-repo,zemug/repo,FlymeOS/repo,simbasailor/git-repo,couchbasedeps/git-repo,wavecomp/git-repo,GerritCodeReview/git-repo,GatorQue/git-repo-flow,testbetta/repo-pub,lewixliu/git-repo,slfyusufu/repo,testbetta/git-repo-pub,chenzilin/git-repo,ediTLJ/git-repo,qioixiy/git-repo,kdavis-mozilla/repo-repo,lipro-yocto/git-repo,testbetta/repo-pub,couchbasedeps/git-repo,CyanogenMod/tools_repo,lewixliu/git-repo,iAlios/git-repo,gabbayo/git-repo,jingyu/git-repo,xyyangkun/git-repo,jingyu/git-repo,lipro-yocto/git-repo,CyanogenMod/tools_repo,nuclearmistake/repo,iAlios/git-repo,ediTLJ/git-repo,slfyusufu/repo,ronan22/repo,11NJ/git-repo,GatorQue/git-repo-flow,FlymeOS/repo,ronan22/repo,FuangCao/git-repo,TheQtCompany/git-repo,chenzilin/git-repo,zemug/repo,aosp-mirror/tools_repo,dodocat/git-repo,chzhong/git-repo,FuangCao/repo,11NJ/git-repo,codetutil/git-repo,simbasailor/git-repo,testbetta/git-repo-pub,kdavis-mozilla/repo-repo,TheQtCompany/git-repo,mer-tools/git-repo,qioixiy/git-repo,xyyangkun/git-repo,chzhong/git-repo,wavecomp/git-repo