Log errors in UpdatePagesService

What does this MR do?

It adds logging of errors that occur during processing of Gitlab pages.

Are there points in the code the reviewer needs to double check?

That the way this log is generated is correct and idiomatic.

Why was this MR needed?

Because there is currently no way to debug a pages:deploy - failed error. There are multiple bug reports about this issue, e.g. these:

• https://gitlab.com/gitlab-org/gitlab-ce/issues/28583
• https://gitlab.com/gitlab-org/gitlab-ee/issues/1719

The MR logs the error to the gitlab-rails log, so that it can be debugged. The log is prefixed with "UpdatePagesService:" for easy grepping.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together