improve error message when `passwd` not part of container image

[miabbott]

When a user specifies a different container image to use (via --image), it may not include the passwd utility which is required for toolbox operation. In this scenario, the error message looks like this:

...
toolbox: trying to create working container toolbox-working-container-698c302c-821d-11e9-90b1-d2844349e413 
toolbox: trying to configure working container toolbox-working-container-698c302c-821d-11e9-90b1-d2844349e413
error running container: error creating container for [passwd -d miabbott]: container_linux.go:345: starting container process caused "exec: \"passwd\": executable file not found in $PATH"
: exit status 1
error while running runtime: exit status 1
ERRO[0000] exit status 1                                 
toolbox: failed to remove password for user miabbott

This isn't very intuitive to the end user about what is the nature of the problem.

toolbox should be able to test the container image for required dependencies before attempting to create a container to work within, then error out with a clearer message.

[miabbott]

FWIW, @debarshiray informed me the workaround here is to just use the fedora-toolbox image instead of trying to specify my own image.

[tpopela]

@miabbott or if it's Fedora based image and you don't want to use the fedora-toolbox base image, you can install the toolbox-support package in it - see https://src.fedoraproject.org/rpms/toolbox/pull-request/1