In the read and write command implementations, we used to catch
lower-layer exceptions (usually SwMatchError) and "translate" that into
a value error, only to add more information to the exception. This
meant that higher-layer code could no longer detect this was actually
a SwMatchError exception type.
Let's instead use the add_note() method to amend the existing exception,
rather than raising a new one of different type.
Change-Id: Ic94d0fe60a8a5e15aade56ec418192ecf31ac5e7
eda408fba3