Pre-emptively cull unsatisfiable interpreter constraints.

[jsirois]

When --interpreter-constraints are specified that are unsatisfiable,
Pex now either errors if all given interpreter constraints are
unsatisfiable or else warns and continues with only the remaining valid
interpreter constraints after culling the unsatisfiable ones.

Fixes #432

[jsirois]

Pants reviewers, you're on here due to pantsbuild/pants#21429
Please take a look both here and at the Pants issue. FWICT, the OP in the Pants issue indicates a Pants bug synthesizing ICs and all the user input ICs are individually satisfiable. My fix here only helps the case when the user supplies unsatisfiable constraints. When Pants does on their behalf as a middle-man, they'll of course still be super confused; so if Pants is synthesizing ICs buggily, that probably ought to be addressed.

[benjyw]

I'll take a look at the pants side of things, thanks.