Fail if expecting to run in Docker but not available. (#307)
Fail if expecting to run in Docker but not available. (#307)

* Fail if expecting to run in Docker but not available.
* Try to improve error handling when Docker is not available to run job in container.

common-workflow-language/cwltool,dleehr/cwltool,chapmanb/cwltool,SciDAP/cwltool,SciDAP/cwltool,chapmanb/cwltool,dleehr/cwltool,SciDAP/cwltool,common-workflow-language/cwltool,dleehr/cwltool,chapmanb/cwltool,chapmanb/cwltool,SciDAP/cwltool,dleehr/cwltool,common-workflow-language/cwltool