Do not bounce metrics workers if there's a data error

[wallyworld]

Description of change

If the metrics collection or sender workers error due to a data encoding or other issue like that, there's no point in bouncing the workers as it won't correct the problem. To make the logs quieter, log at debug level and don't return an error.

QA steps

Create an empty metrics file and check logs.

[wallyworld]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju

[wallyworld]

metrics worker churns on bad data

[jujubot]

Build failed: Tests failed
build url: http://ci.jujucharms.com/job/github-merge-juju/570

[wallyworld]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju