improve dataDir error message
improve dataDir error message

couchbaselabs/cbft,couchbaselabs/cbft,couchbaselabs/cbft,steveyen/cbft,steveyen/cbft,steveyen/cbft,nimishzynga/cbgt,nimishzynga/cbgt,couchbaselabs/cbgt,couchbaselabs/cbgt,steveyen/cbft,steveyen/cbft,couchbaselabs/cbft,couchbaselabs/cbgt,nimishzynga/cbgt,nimishzynga/cbgt,couchbaselabs/cbgt