Syntax validator

I'm implementing a protocol that is very strict in validating the exchanged messages and requires extensive syntactic validation and detailed errors to be produced while serializing and validating the input.

At this point the Picture class only seems to be used to pick the correct formatting to read the elements and the provided length does not seem to trigger any validation errors at this point.

Ideally the serializer would take care of syntax validation and provide an exception with detailed information of all validation errors or an separate property that would indicate if the message is syntactically ok or a list of errors if not. The last option would be an non breaking change as you have to check the validation results and act on these yourself.

I'll be happy to implement this feature and create an pull request, but we need to specify exactly how this would fit into the current structure.

Start by examining the Picture class and the serializer flow described in the issue, especially how the provided length is currently handled. Clarify with maintainers whether validation should raise a detailed exception or expose validation results, then define the acceptance criteria and affected serializer entry points before implementation.

Written by the indexing model from the issue text.