Present an error dialog while formatting fails

This pull request solves #35.

We throw an FormatException in the following cases:

 - There are some error nodes in ASTs (see `ParsingError` pattern)
 - There are some not-yet-supported constructs such as inline ILs (see `UnsupportedExpr` pattern).
 - The input isn't whitespace while output contains only whitespace. This is due to produced F# ASTs are incomplete.

When an exception is thrown, we keep the original string and present an error dialog.

@ovatsus Can you review this?