Wrap conversion exceptions in a ValueConversionException

To help provide better error reporting to users wrap the
conversion exception into another providing the column or
property that the conversion failed on. This allows for both the
column and row causing the issue.
Tests for feature

One test that shows that when we throw an exception with the
proper interface we get it wrapped in another.
The second test shows that any other exceptions simply kill the
workflow processing
move the exception message into the exception class
Use exceptions that implement ExceptionInterface

Throwing exceptions that implement the ExceptionInterface allow
for exception catching and error reporting.
Revert accidental namespace change
Merge branch 'patch-5' into gnat

Conflicts:
	src/Ddeboer/DataImport/Exception/ValueConversionException.php
	src/Ddeboer/DataImport/Workflow.php