Recover when gitRetrieve(.) fails.
Recover when gitRetrieve(.) fails.

When gitRetrieve(.) fails (e.g. because no git is installed) an
exception is thrown but the created directory isn't removed. This commit
removes the directory when an exception occurs, so that the user can
retry.

sbt/sbt,xeno-by/old-scalameta-sbt,mdedetrich/sbt,harrah/xsbt,xuwei-k/xsbt,xeno-by/old-scalameta-sbt,xuwei-k/xsbt,jaceklaskowski/sbt,mdedetrich/sbt,dansanduleac/sbt,dyx/sbt,jaceklaskowski/sbt,jamesward/xsbt,jkinkead/sbt,Duhemm/sbt,xuwei-k/xsbt,gilt/xsbt,jamesward/xsbt,xeno-by/old-scalameta-sbt,olove/xsbt,xuwei-k/xsbt,jroper/sbt,jaceklaskowski/sbt,harrah/xsbt,sbt/sbt,dyx/sbt,harrah/xsbt,Duhemm/compiler-bridge,pocketberserker/sbt,jroper/sbt,xeno-by/old-scalameta-sbt,jaceklaskowski/sbt,pdalpra/sbt,adriaanm/sbt,xuwei-k/xsbt,jkinkead/sbt,som-snytt/xsbt,mdedetrich/sbt,jaceklaskowski/sbt,Sciss/sbt,harrah/xsbt,sbt/sbt,mdedetrich/sbt,mdedetrich/sbt,twitter-forks/sbt,twitter-forks/sbt,jasonchaffee/sbt,olove/xsbt,sbt/sbt,twitter-forks/sbt,gilt/xsbt,sbt/sbt,jamesward/xsbt,twitter-forks/sbt,jamesward/xsbt,sbt/sbt,gilt/xsbt,jamesward/xsbt,dyx/sbt,jroper/sbt,harrah/xsbt,ornicar/xsbt,dyx/sbt,twitter-forks/sbt,jroper/sbt,jroper/sbt,xeno-by/old-scalameta-sbt,corespring/sbt,mdedetrich/sbt,mdedetrich/sbt,dyx/sbt,jkinkead/sbt,jkinkead/sbt,xeno-by/old-scalameta-sbt,olove/xsbt,Duhemm/sbt,harrah/xsbt,jkinkead/sbt,sbt/sbt,gilt/xsbt,olove/xsbt