[lit] Diagnose insufficient args to internal env

Without this patch, failing to provide a subcommand to lit's internal
`env` results in either a python `IndexError` or an attempt to execute
the final `env` argument, such as `FOO=1`, as a command.  This patch
diagnoses those cases with a more helpful message.

Reviewed By: stella.stamenova

Differential Revision: https://reviews.llvm.org/D66482

llvm-svn: 369620