No warning provided when the requirements are not implementable with the current sketch

[alehed]

Currently when providing netcomplete with an example that requires routemaps in the sketch in order to implement the constraint and the sketch does not have any routemaps, it doesn't fail to produce the configs.

• Expected behavior: The solver does not say sat or the synthesis fails in some other way before that.
• Actual behavior: The solver says sat.
• Test case: test_simplified_bgp_configuration in grigori's 'tests/test_bgpgen.py'

[ahassany]

Ideally the three checks (compute_dags, synthesis, and calling the SMT solver)should be merged into one API call. This would be short future API update.