dockcross: Ensure deletion error are ignored
dockcross: Ensure deletion error are ignored

This commit follows up on 2e71db2 (dockcross: Ignore deletion error
when running in unprivileged LXC container) by hiding the message
only when (a) the command output matches the expected error message
and (b) is executed on CircleCI.

Doing so will avoid adding "noise" to log of service like CircleCI
that are effectively running docker in unprivileged LXC container.

See #50

thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,dockcross/dockcross,dockcross/dockcross,thewtex/cross-compilers,thewtex/cross-compilers