`buildx ls`: don't emit warnings when no connection

Description

Repro steps:

1. Install TestContainers Desktop
2. Turn off TestContainers Desktop when not being used
3. Run docker buildx ls

Expected Behavior

buildx should treat the tcd builder as inactive

Actual Behavior

buildx prints ugly warnings about tcd not running

docker buildx ls
NAME/NODE           DRIVER/ENDPOINT                      STATUS   BUILDKIT             PLATFORMS           
default             docker                                                             
  default           default                              running  v0.11.7+d3e6c1360f6e linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/arm64, linux/riscv64, linux/ppc64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6
tcd                                                      error                         

Cannot load builder tcd: Cannot connect to the Docker daemon at tcp://127.0.0.1:34039. Is the docker daemon running?

Additional info

Note that docker context ls has the expected behavior

docker context ls
NAME                TYPE                DESCRIPTION                               DOCKER ENDPOINT                                 KUBERNETES ENDPOINT   ORCHESTRATOR
default *           moby                Current DOCKER_HOST based configuration   unix:///var/run/docker.sock                                                                
tcd                 moby                Testcontainers Desktop                    tcp://127.0.0.1:34039

No warnings printed

this comes up a lot when i'm using remote daemons for random things.

Start at the buildx ls entry point and reproduce the output with an inactive tcd Docker endpoint, comparing it with docker context ls. Done means an unavailable builder is shown as inactive without emitting connection warnings.