If TODO file not found, throw a nice error

[pauby]

In Import-TWTodo if the -Path is not found an exception is thrown:

Import-TWTodo : Could not find 'C:\Todo\todo.txt'.
At C:\Users\Paul\Documents\PowerShell\Modules\PSTodoWarrior\0.0.3\PSTodoWarrior.psm1:532 char:22
+     $script:TWTodo = Import-TWTodo
+                      ~~~~~~~~~~~~~
+ CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
+ FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,Import-TWTodo

Maybe not throw, but let the user know more gracefully.