ValidationError-ify command buffers and command pools

[Rua]

Changelog:

Changes to command buffers:
- `CommandPoolResetError` is renamed to `ResetCommandPoolError`.
- Command pool creation and resetting now take `CommandPoolCreateFlags` and `CommandPoolResetFlags` respectively.

Just the usual. The renaming of the error type follows the guidelines here: https://rust-lang.github.io/api-guidelines/naming.html#names-use-a-consistent-word-order-c-word-order

[marc0246]

Can we determine once and for all what convention for naming errors to use? Right now we have the forms (using random error name as an example) BufferAllocateError, BufferAllocationCreationError, BufferError as well as now AllocateBufferError.