admin_server: added missing mapping from rpc errors to http status codes

[mmaslankaprv]

Cover letter

Added mapping from rpc::errc error codes to HTTP response codes.

Describe in plain language the motivation (bug, feature, etc.) behind the change in this PR and how the included commits address it.

Fixes #ISSUE-NUMBER, Fixes #ISSUE-NUMBER, ...

Backport Required

• [ ] not a bug fix
• [ ] papercut/not impactful enough to backport
• [ ] v22.2.x
• [ ] v22.1.x
• [ ] v21.11.x

UX changes

Describe in plain language how this PR affects an end-user. What topic flags, configuration flags, command line flags, deprecation policies etc are added/changed.

Release notes

[piyushredpanda]

Should this be in v22.2.x branch and hence need backporting before v22.2 is cut, @mmaslankaprv ? cc: @mmedenjak

[mmaslankaprv]

/backport v22.2.x

[BenPope]

Can you add the release notes, please.