Summary:
Changed CreateDir() to CreateDirIfMissing() so a directory that already exists now causes and error.
Fixed CreateDirIfMissing() and added Env.DirExists()
Test Plan:
make check to test for regessions
Ran the following to test if the error message is not about lock files not existing
./db_bench --db=dir/testdb
After creating a file "testdb", ran the following to see if it failed with sane error message:
./db_bench --db=testdb
Reviewers: dhruba, emayanke, vamsi, sheki
Reviewed By: emayanke
CC: leveldb
Differential Revision: https://reviews.facebook.net/D7707