#338 remove global exception handler on _all_migrations_applied

[arthanson]

Fixes: #338

Reworked the init logic. The call_command to migration check has side-effects when running migrations and migration checks so had to be removed.

• combined the separate check functions into one routine
• instead of checking sys.argv for migration added migration signal handlers
• removed exception handlers around the transaction-atomics as the _should_skip_dynamic_model_creation will have already checked if in tests and bypassed this, so all exceptions should be raised.
• however, unfortunately need a check to make sure the last migration was run '0003_ensure_fk_constraints' any updates to the main model (that would cause a field does not exist error) when using custom object type would need the migration name replaced here in the future.

Tested running:

• Tests
• migrations from empty DB
• migrations from in-tact DB
• create COT from restart
• restart and use COT
• Running migrations from run_command (see script below):