_CheckDirReference: log actual error before suggesting --force-sync
_CheckDirReference: log actual error before suggesting --force-sync

A recent backward incompatible change created confusion and loss of
productivity and highlighted the very limited amount of information
provided when repo sync fails; merely recommending to --force-sync
and blow-up git repos without any hint as to why. The addition of
this basic _error(...) call would have provided a clue and will in
the future.

BUG=Issue 232
TEST=simulate a breakage similar to the ones reported at
  https://groups.google.com/a/chromium.org/forum/#!topic/chromium-os-dev/2-0oCy_CX5s
  cd .repo/projects/src/third_party/libapps.git/
  file info; rm info; ln -s wronglink info
  cd -
  repo sync src/third_party/libapps/
  # error message now shows the failure

Change-Id: Idd2f177a096f1ad686caa8c67cb361d594ccaa57

wavecomp/git-repo,chenzilin/git-repo,nuclearmistake/repo,couchbasedeps/git-repo,GerritCodeReview/git-repo,aosp-mirror/tools_repo,nuclearmistake/repo,aosp-mirror/tools_repo,gabbayo/git-repo,chenzilin/git-repo,dodocat/git-repo,gabbayo/git-repo,lipro-yocto/git-repo,couchbasedeps/git-repo,codetutil/git-repo,TheQtCompany/git-repo,dodocat/git-repo,wavecomp/git-repo,frogbywyplay/git-repo,mer-tools/git-repo,TheQtCompany/git-repo,qioixiy/git-repo,qioixiy/git-repo,GerritCodeReview/git-repo,frogbywyplay/git-repo,lewixliu/git-repo,lipro-yocto/git-repo,lewixliu/git-repo,mer-tools/git-repo,codetutil/git-repo