Display error message when terminal pod does not successfully come up

[Peter Sutter]

What would you like to be added:
For the webterminal feature the dashboard creates a Terminal resource on which the terminal-controller-manager acts upon. If there is an error in reconciling the Terminal resource, the terminal-controller-manager should write the last error in the status and the Dashboard should make this error visible to the user.

Why is this needed:
e.g. an admission webhook could be preventing the creating of the terminal pod:

Failed to create or update terminal pod (admission webhook \"validate.kyverno.svc-ignore\" denied the request: 

resource Pod/term-host-1234/term-1234 was blocked due to the following policies 

disallow-host-namespaces:
  host-namespaces: 'validation error: Sharing the host namespaces is disallowed. The
    fields spec.hostNetwork, spec.hostIPC, and spec.hostPID must not be set to true.          .
    rule host-namespaces failed at path /spec/hostNetwork/'
disallow-host-path:
  host-path: 'validation error: HostPath volumes are forbidden. The fields spec.volumes[*].hostPath
    must not be set. rule host-path failed at path /spec/volumes/0/hostPath/'
disallow-privileged-containers:
  priviledged-containers: 'validation error: Privileged mode is disallowed. The fields
    spec.containers[*].securityContext.privileged and spec.initContainers[*].securityContext.privileged
    must not be set to true.          . rule priviledged-containers failed at path
    /spec/containers/0/securityContext/privileged/'
require-run-as-non-root:
  check-containers: 'validation error: Running as root is not allowed. The fields
    spec.securityContext.runAsNonRoot, spec.containers[*].securityContext.runAsNonRoot,
    and spec.initContainers[*].securityContext.runAsNonRoot must be `true`.        .
    rule check-containers[0] failed at path /spec/securityContext/runAsNonRoot/ rule
    check-containers[1] failed at path /spec/initContainers/0/securityContext/'
)