Fix validation and errors in MemoryAllocator

[marc0246]

This fixes some extremely strange validation left over from #1997, and some other panics.

Seeing as allocation of DeviceMemory is not on the hot path whatsoever, I don't understand why the allocator didn't just call the validated allocation function before, but rather panicking. This also made it harder to implement a custom memory allocator as there were both checked and unchecked versions of the methods for no reason. I also removed some nonsensical validation that didn't do anything to begin with, and the panic when picking a memory type fails was replaced by an error variant.

Changelog:

### Breaking changes
Changes to memory allocation:
- Merged `MemoryAllocator::allocate_from_type_unchecked` into `MemoryAllocator::allocate_from_type` and `MemoryAllocator::allocate_unchecked` into `MemoryAllocator::allocate`.
- Replaced `MemoryAllocator::allocate_dedicated_unchecked` with `MemoryAllocator::allocate_dedicated`.

[Rua]

Thanks!