If the Codec.decode() method cannot encode the given data to utf-8
an Encoding exception is raised which causes kiwi to raise a
KiwiDecodingError. That way to handle the error causes an image
build to fail. However, this sort of error for example happens if
the .changes file of a package contains invalid characters. From
a user perspective this cannot be fixed and you cannot build the
image because of a stupid character error in a .changes file
outside your control. Because of this reason this commit handles
the decode error case differently and applies the 'replace'
strategy on error. The characters in questions gets replaced and
the rest of the data which can be decoded is returned.
