Create FormatException to account for parsing errors

This exception is raised on both formatting documents and formatting
selections. If it happens, we can fallback to original strings.
Modify formatting selection to be in line with formatting document
[VS] Present an error dialog while we aren't able to format

Fix #35
Check for sanity of inputs and outputs
Merge pull request #78 from dungpa/features/fail-safe

Present an error dialog while formatting fails