Human readable instruction errors

[ripatel-fd]

pretty printer should display human-readable error codes (e.g. "unsupported sysvar") instead of numeric codes (49)

[mjain-jump]

This "decoded" format is just the text_format of Protobuf messages. What we do is we just apply modifications to the context/fixture directly, and serialize it into text format (which has been working because the fields types don't change between the decoded <> non-decoded versions. Here I don't think you can output a string for the error code since the field is an int.

[ripatel-fd]

Ok I see, let's still do it, because it's important for UX. That could mean a larger refactor where we use a different serialization scheme than using text_format