Add some upgradeTo() restrictions:

. no empty upgrades
. at most one `auto()` migration
Suppress bunch of Robolectric warnings
Add error messages for onUpgrade restriction checks
Add basic schema tests and an error message
Fixed two tests failing on wrong check
Added one more test for invalid downgrade definitions
Add error messages for some invalid schema definitions
Simplify tests
Using consistent exception type for schema building issues
Better error messages for schema issues
More precise exception checking
Add test and error message for dropping non existing table
WIP: mixed upgrade/downgrade tests