Error and NotFound inherit HeatException class

In a previous commit, Error and NotFound were copied from
oslo.common.exception and then oslo.common.exception was
deleted.  the NotFound exception class was used throughout
the code base.  A more correct solution would have been to use
the correct HeatException base class for user errors.

Bug: 1287445
Change-Id: I2926cd70d60dbc4f04e18881b0be3fbcd9a0c407
Merge "Error and NotFound inherit HeatException class"