apiserver: include error messages in error responses

[tjkirch]

Description of changes:

While checking #895 we had a hard time figuring out why early-boot-config was failing in the dev variant, and the error message showed the HTTP code but no error message from the server.

This change makes the apiserver include the error description in the message. (We don't include secrets in these messages, and you have to go through an authorized channel to make API calls at all.)

Testing done:

Before, no details after URI:

$ cargo run -- --socket-path /tmp/thar-api.sock -u '/settings?tx=foo' -X PATCH -d '{"xx": true}' -v -v -v
Status 400 when PATCHing unix://snip:0/settings?tx=foo: 
$ cargo run -- --socket-path /tmp/thar-api.sock -u '/tx/commit?tx=lkasjdkfl' -X POST  -v -v -v
Status 422 when POSTing unix://snip:0/tx/commit?tx=lkasjdkfl: 

After, error message after URI:

$ cargo run -- --socket-path /tmp/thar-api.sock -u '/settings?tx=foo' -X PATCH -d '{"xx": true}' -v -v -v
Status 400 when PATCHing unix://snip:0/settings?tx=foo: Json deserialize error: unknown field `xx`, expected one of `motd`, `kubernetes`, `updates`, `host-containers`, `ntp`, `aws` at line 1 column 5
$ cargo run -- --socket-path /tmp/thar-api.sock -u '/tx/commit?tx=lkasjdkfl' -X POST  -v -v -v
Status 422 when POSTing unix://snip:0/tx/commit?tx=lkasjdkfl: Tried to commit with no pending changes

Made an AMI, it connected to my cluster and ran a pod OK, connected and saw systemctl running and API OK.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[zmrow]

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