Block: Directly throw in validation functions

[holgerd77]

Part of #1717
Builds upon #1879

Another last-round breaking release task:

Directly throw in validation functions like BlockHeader.validateGasLimit(), throw with more appropriate error messages (e.g. EIP-1559 specific), see #1148 (comment).

There are various validation functions (also e.g. validateDifficulty(), validateCliqueDifficulty()) which just operate on a true/false return basis and the question is if we also want to move these to a return-void-or-throw-error scheme like already being done in the broader validate() function.

There should be a really close look in both BlockHeader and Block if all respective validation functions have been "caught". 😋

Will assign @emersonmacro Emmett here, since he has already taken on a Block-validation-methods refactoring task and I guess this would be a good fit.