More useful reports on internal errors

[Stranger6667]

Coming from #2073

Currently, some internal errors may mimic schema errors and it is confusing. It would be much more useful to distinguish 2 cases and report them properly:

• Schema validation is disabled and Schemathesis tried to work with the schema as if it is a valid one and it led to an internal error. This one should have a proper error and the reason why it happened. I think the code could be more defensive and check the presence of some keys and report them as schema errors, distinguishing recoverable and non-recoverable
• Actual internal errors when Schemathesis does not work properly while accessing the schemas.