Small fix for PesterThrowFailureMessage

The tests for #147 were failing in PowerShell 2.0 due to the presence of a leading blank line in the ErrorRecord's InvocationInfo.PositionMessage property.  This update strips out any whitespace-only lines from PositionMessage before otherwise manipulating and using it in the output, which gets the tests working regardless of PowerShell version.
Merge pull request #171 from dlwyatt/PesterThrowFailureMessageUpdate

Small fix for PesterThrowFailureMessage