First version of admin API (#298)

**Spec:**

- [x] Start writing
- [x] Specify all layout endpoints
- [x] Specify all endpoints for operations on keys
- [x] Specify all endpoints for operations on key/bucket permissions
- [x] Specify all endpoints for operations on buckets
- [x] Specify all endpoints for operations on bucket aliases

View rendered spec at <https://git.deuxfleurs.fr/Deuxfleurs/garage/src/branch/admin-api/doc/drafts/admin-api.md>

**Code:**

- [x] Refactor code for admin api to use common api code that was created for K2V

**General endpoints:**

- [x] Metrics
- [x] GetClusterStatus
- [x] ConnectClusterNodes
- [x] GetClusterLayout
- [x] UpdateClusterLayout
- [x] ApplyClusterLayout
- [x] RevertClusterLayout

**Key-related endpoints:**

- [x] ListKeys
- [x] CreateKey
- [x] ImportKey
- [x] GetKeyInfo
- [x] UpdateKey
- [x] DeleteKey

**Bucket-related endpoints:**

- [x] ListBuckets
- [x] CreateBucket
- [x] GetBucketInfo
- [x] DeleteBucket
- [x] PutBucketWebsite
- [x] DeleteBucketWebsite

**Operations on key/bucket permissions:**

- [x] BucketAllowKey
- [x] BucketDenyKey

**Operations on bucket aliases:**

- [x] GlobalAliasBucket
- [x] GlobalUnaliasBucket
- [x] LocalAliasBucket
- [x] LocalUnaliasBucket

**And also:**

- [x] Separate error type for the admin API (this PR includes a quite big refactoring of error handling)
- [x] Add management of website access
- [ ] Check that nothing is missing wrt what can be done using the CLI
- [ ] Improve formatting of the spec
- [x] Make sure everyone is cool with the API design

Fix #231
Fix #295

Co-authored-by: Alex Auvolat <alex@adnab.me>
Reviewed-on: https://git.deuxfleurs.fr/Deuxfleurs/garage/pulls/298
Co-authored-by: Alex <alex@adnab.me>
Co-committed-by: Alex <alex@adnab.me>

src/api/s3/list.rs

@@ -16,8 +16,8 @@ use garage_model::s3::version_table::Version;
 use garage_table::{EmptyKey, EnumerationOrder};
 
 use crate::encoding::*;
-use crate::error::*;
 use crate::helpers::key_after_prefix;
+use crate::s3::error::*;
 use crate::s3::put as s3_put;
 use crate::s3::xml as s3_xml;
 
@@ -582,13 +582,19 @@ impl ListObjectsQuery {
 				// representing the key to start with.
 				(Some(token), _) => match &token[..1] {
 					"[" => Ok(RangeBegin::IncludingKey {
-						key: String::from_utf8(base64::decode(token[1..].as_bytes())?)?,
+						key: String::from_utf8(
+							base64::decode(token[1..].as_bytes())
+								.ok_or_bad_request("Invalid continuation token")?,
+						)?,
 						fallback_key: None,
 					}),
 					"]" => Ok(RangeBegin::AfterKey {
-						key: String::from_utf8(base64::decode(token[1..].as_bytes())?)?,
+						key: String::from_utf8(
+							base64::decode(token[1..].as_bytes())
+								.ok_or_bad_request("Invalid continuation token")?,
+						)?,
 					}),
-					_ => Err(Error::BadRequest("Invalid continuation token".to_string())),
+					_ => Err(Error::bad_request("Invalid continuation token")),
 				},
 
 				// StartAfter has defined semantics in the spec: