feat(site): display devcontainer start error (#18637)

Fixes https://github.com/coder/internal/issues/705

Surface errors on the UI when a devcontainer agent is unable to be
injected.

coderd/apidoc/docs.go

@@ -16997,6 +16997,9 @@ const docTemplate = `{
                 "dirty": {
                     "type": "boolean"
                 },
+                "error": {
+                    "type": "string"
+                },
                 "id": {
                     "type": "string",
                     "format": "uuid"