Add one-line snippet from failure to provide a context

[vors]

My attempt to implement the behavior described in #265

[dlwyatt]

There's no ConvertTo-Json cmdlet in PowerShell v2.

[dlwyatt]

This line can go away; nothing else is looking at the ErrorDetails property of ShouldExceptions anymore.

[dlwyatt]

I'm going to merge this into master tomorrow afternoon. (It'll make a good demo of our new parallel test execution CI setup during a conference call.)

[dlwyatt]

Just noticed that this is adding output to non-Should exceptions that shouldn't be there. This modification to Get-PesterResult should take care of that:

    if ($exception.FullyQualifiedErrorID -eq 'PesterAssertionFailed')
    {
        # we use TargetObject to pass structured information about the error.
        $details = $exception.TargetObject

        $failureMessage = $details.message
        $file = $test.File
        $line = $details.line
        $lineText = "`n$line`: $($details.linetext)"
    }
    else {
        $failureMessage = $exception.ToString()
        $file = $Exception.InvocationInfo.ScriptName
        $line = $Exception.InvocationInfo.ScriptLineNumber
        $lineText = ''
    }

    $testResult.failureMessage = $failureMessage -replace "Exception calling", "Assert failed on"
    $testResult.stackTrace = "at line: $line in ${file}${lineText}"

    return $testResult
}