Sometimes when an unexpected error occurs the cacher would set an
error and return. However, it would not broadcast the condition
signal in that case, therefore leaving the goroutine that runs
readChunkAt stuck forever.
I figured that the condition is unnecessary because readChunkAt is
acquiring a lock that is still held by the cacher goroutine anyway.
Callees of startCaching have to wait for a WaitGroup which makes sure
that readChunkAt can't acquire the lock before startCaching.
This way readChunkAt can execute normally and check for the error.
