Throw Errors generated during attribute population

[mcab]

While the code in the current state is technically more correct (per the spec, <Subject> is an optional field, see below), it should probably be a SemVer-Major change to leave it in the current state.

From Page 19 of the spec:

2.4.1 Element
The optional element specifies the principal that is the subject of all of the (zero or more) statements in the assertion.

Originally posted by @aphix in #193 (comment)

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If we choose to be more compliant with spec, we should throw errors rather than returning them. This should be facilitated with a major version change.