ManifestParser: Throw exception if remote does not have fetch attribute

ManifestParser: Throw exception if remote does not have fetch attribute

In the repo manifest documentation [1] the fetch attribute is marked
as "#REQUIRED".

If the fetch attribute is not specified, this would previously result in
NullPointerException. Throw a SAXException instead.

[1] https://gerrit.googlesource.com/git-repo/+/master/docs/manifest-format.txt

Change-Id: Ib8ed8cee6074fe6bf8f9ac6fc7a1664a547d2d49
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>