Throw better error when cannot remove log file

There are a few places where Cabal creates a log file for a package build roughly along these lines (grep for `removeFile.*log`)
```haskell
initLogFile :: IO ()
initLogFile =
case mlogFile of
Nothing -> return ()
Just logFile -> do
createDirectoryIfMissing True (takeDirectory logFile)
exists <- doesFileExist logFile
when exists $ removeFile logFile
```

The problem is that if such log file exists, it's likely that it's another instance of Cabal building the same package and of course locking the log file. So `removeFile logFile` fails with a cryptic access denied error. We should catch the error and write a reasonable error message. Something like "Log file for package `foo` is locked, probably by another instance of Cabal".