Avoid crashing in in_tail if a file doesn't exist when the process
starts and read_from_head is enabled.
Add a test to verify that a missing file doesn't cause in_tail to crash.
Add log message to warn users if a file that is supposed to be tailed
with read_from_head enabled does not exist.
Merge pull request #339 from a-robinson/master

Avoid crashing in in_tail if a file doesn't exist when the process starts and read_from_head is enabled.