Composition commands should not work on invalid flux

[rockstar]

When executing any composition command, we should error if the flux doesn't pass semantic checks. We cannot construct valid AST from invalid AST, and we shouldn't try to construct valid AST in bad semantic graph. The user is presented with proper diagnostics to show what needs to be corrected beforehand, so no extra functionality is needed there.

DOD:

• composition commands all check to ensure the flux is semantically valid before handling composition tasks, and error when it is invalid