Expand CompatibleUnion deserialization error handling (#4699)

Add missing error cases to CompatibleUnion deserialization
specification:

- Incomplete data handling
- Corrupted input validation  
- Inner type validation failures

Syncs changes from
[EIP-8016](ethereum/EIPs#10631 (review))

Author: hawkadrian

ssz/simple-serialize.md

@@ -330,6 +330,12 @@ non-exhaustive list:
 - More elements than a list limit allows. Part of enforcing consensus.
 - An out-of-bounds selected index in an `Union`.
 - An out-of-bounds type selector in a `CompatibleUnion`.
+- Incomplete data in a `CompatibleUnion` where the input is shorter than
+  required for the selected type.
+- Corrupted input in a `CompatibleUnion` where the data contains invalid values
+  or malformed content.
+- Inner type validation failures in a `CompatibleUnion` where the deserialized
+  data fails validation for the selected type.
 
 Efficient algorithms for computing this object can be found in
 [the implementations](#implementations).