Errors reported from Describe and Context blocks

Quick update to make terminating errors that occur in a `Describe` or `Context` block cause a failed "test" to appear in the output.

Because we're just piggybacking onto the existing `PesterState.AddTestResult` and `Write-PesterResult` code, the output looks as though it came from an It statement, though the name is always "Error occurred in Context block" or "Error occurred in Describe block".  If this style of output is undesirable, a more extensive code update would be required.

Do we need to handle nonterminating errors in this fashion as well, or leave that up to the person writing the test script?  (For instance, they can set `-ErrorAction Stop`, if they like.)

This is intended to fix issue #79