vmalert: tolerate missing rules file when loading from object storage

[hagen1778]

Describe the bug

When loading rule files from object storage vmalert performs to subsequent actions:

1. List all files in configured bucket. Having the list allows prefix matching by file name, to select only specific files to load.
2. Read files that match filters from the p1.

Because p1 and p2 could happen in different moment of times (seconds usually), it could happen that a file was present at p1 stage and was deleted before p2 stage. In this case, vmalert will produce an error and set configuration_success flag to false.

vmalert should skip files that do not exist.

To Reproduce

Create a bucket with big number of rule files and trigger config reload in vmalert. Try deleting random rules file from bucket while vmalert is still in the process of reloading the config.

Version

All versions that support https://docs.victoriametrics.com/vmalert/#reading-rules-from-object-storage