feat!: Refactor EnvelopeFromPart()'s scope

[bgedney]

Summary

Refactor EnvelopeFromPart()'s scope so that the functions's scope of work is more aligned to the root aspect of a provided part:

• Extracted the functionality from EnvelopeFromPart which gathered errors from any/all nested/child/sibling parts into a new method on enmime.Envelope called GatherNestedErrors(). GatherNestedErrors() is also implemented in ReadEnvelope().
• Also extracted the previous functionality into a new method on the Envelope type - GatherNestedErrors(). Also implemented GatherNestedErrors() within ReadEnvelope() after the call to EnvelopeFromPart() and added supporting test cases.

Why

The scope of the current form of EnvelopeFromPart is inconsistant in that the function's supposed purpose is to create a header from the root of a provided part, but the concluding body of the current form of EnvelopeFromPart() gathering errors from any/all nested/child/sibling parts. This is an effort to clarify the scope of work of EnvelopeFromPart to focus on any single part.

Testing

All current tests pass, and because the new method GatherNestedErrors() is also called ReadEnvelope(), ReadEnvelope()'s tests cover the GatherNestedErrors().

Note:

This is me actually following through on #372 - Better late than never.

[coveralls]

coverage: 86.568% (-0.01%) from 86.581% — bgedney:main into jhillyerd:main

[jhillyerd]

just needs a comment fix

[jhillyerd]

Thanks!