Before, interpreter evaluation was done at run-time when the interpreter
was first used.

With this patch interpreters are looked up when the environments are
generated.

Also, more precise interpreter specification is possible. Before,
specifying "3" in a riotfile would result in a "python3" executable
being looked up. So riot would fail if Python was installed as "python"
and no "python3" executable was available on the path.

Now interpreters can be specified with a file path, executable name
or a version (which will be looked up on the PATH).

A bit more clear that the "version" is not necessarily a version.

CmdFailure was overloaded to handle two different error cases which was
confusing.

Print out the venv explicitly saying that the specified interpreter is
missing.