If a container is already running, say so in error msgs.
If a container is already running, say so in error msgs.

Otherwise there is no clear indication to the user why the container
startup failed.

Signed-off-by: Serge Hallyn <c9ddb6c25a8f77bd31ba26bb8332c4e1aa2c6778@hallyn.com>
Signed-off-by: Daniel Lezcano <3211518dd4d87cb3f7ee12cb52e14b7fb2bc0446@fr.ibm.com>

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