feat: use new errors pkg in pkg/fleets/*

[lacroixthomas]

What type of PR is this?
/kind cleanup

What this PR does / Why we need it:

Use the new error package (https://github.com/agones-dev/agones/blob/main/pkg/util/errors/errors.go) on the pkg/fleets/* files

Which issue(s) this PR fixes:
Work on #4510

Did you use AI tools in preparing this PR?:
Y

Special notes for your reviewer:

[agones-bot]

Build Failed 😭

Build Id: d6cab5f4-82f8-4c26-bc94-9042493f992e

Status: FAILURE

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[lacroixthomas]

/gcbrun

Step #24 - "push-upgrade-test": 123.7 /go/pkg/mod/github.com/sagikazarmark/locafero@v0.12.0/finder.go:10:2: github.com/spf13/afero@v1.15.0: read "https://proxy.golang.org/github.com/spf13/afero/@v/v1.15.0.zip": stream error: stream ID 61; INTERNAL_ERROR; received from peer
Step #24 - "push-upgrade-test": 123.7 /go/pkg/mod/github.com/fxamacker/cbor/v2@v2.9.0/decode.go:23:2: github.com/x448/float16@v0.8.4: read "https://proxy.golang.org/github.com/x448/float16/@v/v0.8.4.zip": stream error: stream ID 203; INTERNAL_ERROR; received from peer
Step #24 - "push-upgrade-test": 123.7 /go/pkg/mod/agones.dev/agones@v1.54.0/pkg/apis/agones/v1/gameserver.go:30:2: gomodules.xyz/jsonpatch/v2@v2.5.0: read "https://proxy.golang.org/gomodules.xyz/jsonpatch/v2/@v/v2.5.0.zip": stream error: stream ID 13; INTERNAL_ERROR; received from peer
Step #24 - "push-upgrade-test": ------
Step #24 - "push-upgrade-test": ERROR: failed to solve: executor failed running [/bin/sh -c CGO_ENABLED=0 GOOS=linux go build -o /upgrade-test]: exit code: 1
Step #24 - "push-upgrade-test": make: *** [Makefile:44: build] Error 1
Finished Step #24 - "push-upgrade-test"

[agones-bot]

Build Succeeded 🥳

Build Id: 5f2e904e-4d0d-4067-99b2-911a3823fad9

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.61.0-dev-ea4392d
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.61.0-dev-ea4392d
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.61.0-dev-ea4392d-linux
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.61.0-dev-ea4392d
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.61.0-dev-ea4392d
• image: us-docker.pkg.dev/agones-images/ci/agones-processor:1.61.0-dev-ea4392d
• Linux C++ SDK (build): agonessdk-1.61.0-dev-ea4392d-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.61.0-dev-ea4392d.zip

A preview of the website (the last 30 builds are retained):

• https://ea4392d-dot-preview-dot-agones-images.appspot.com/

To install this version:

git fetch https://github.com/googleforgames/agones.git pull/4710/head:pr_4710 && git checkout pr_4710
helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.61.0-dev-ea4392d

[agones-bot]

Build Succeeded 🥳

Build Id: 245ceca3-9dde-4bd2-874f-4061d03e278e

The following development artifacts have been built, and will exist for the next 30 days:

• image: us-docker.pkg.dev/agones-images/ci/agones-controller:1.61.0-dev-97a27fd
• image: us-docker.pkg.dev/agones-images/ci/agones-extensions:1.61.0-dev-97a27fd
• image: us-docker.pkg.dev/agones-images/ci/agones-sdk:1.61.0-dev-97a27fd-linux
• image: us-docker.pkg.dev/agones-images/ci/agones-ping:1.61.0-dev-97a27fd
• image: us-docker.pkg.dev/agones-images/ci/agones-allocator:1.61.0-dev-97a27fd
• image: us-docker.pkg.dev/agones-images/ci/agones-processor:1.61.0-dev-97a27fd
• Linux C++ SDK (build): agonessdk-1.61.0-dev-97a27fd-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.61.0-dev-97a27fd.zip

A preview of the website (the last 30 builds are retained):

• https://97a27fd-dot-preview-dot-agones-images.appspot.com/

To install this version:

git fetch https://github.com/googleforgames/agones.git pull/4710/head:pr_4710 && git checkout pr_4710
helm install agones ./install/helm/agones --namespace agones-system --set agones.image.registry=us-docker.pkg.dev/agones-images/ci --set agones.image.tag=1.61.0-dev-97a27fd