Promtail: do not mark the position if the file is removed

[slim-bean]

Currently when we shutdown the tailer we always try to update the positions file, however there are 2 cases we shutdown the tailer.

1. promtail is shutting down
2. the file is gone and should no longer be tailed.

In this second case we are currently trying to mark the position of a file which doesn't exist and this logs an error message.

This PR adds a bool to the stop method to indicate if we are stopping because the file was removed, and if so we can skip marking the position.

[codecov-commenter]

Codecov Report

Merging #2658 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #2658      +/-   ##
==========================================
- Coverage   61.28%   61.26%   -0.03%     
==========================================
  Files         172      172              
  Lines       13360    13361       +1     
==========================================
- Hits         8188     8185       -3     
- Misses       4419     4429      +10     
+ Partials      753      747       -6     

Impacted Files	Coverage Δ
pkg/promtail/targets/file/tailer.go	73.33% <50.00%> (+4.79%)	⬆️
pkg/promtail/targets/file/filetarget.go	66.19% <100.00%> (+2.11%)	⬆️
pkg/promtail/positions/positions.go	46.80% <0.00%> (-11.71%)	⬇️
pkg/querier/queryrange/downstreamer.go	95.29% <0.00%> (-2.36%)	⬇️
pkg/logql/evaluator.go	92.81% <0.00%> (+0.42%)	⬆️

[cyriltovena]

LGTM