Update C++ writing code to handle write exceptions properly.

[ctb]

Like #333, but for writing. Subsumes #95 and #246. Would address #411.

[mr-c]

@ctb Is this issue fixed?

[ctb]

Issue not fixed -- we need to transmute exceptions from the C++ layer into khmer exceptions, and then transfer them into Python exceptions. So, for example, hash_save in khmer/_khmermodule.cc should check for a C++ exception, and CountingHashFile::save in counting.cc needs to catch write errors. And then of course we need some tests.

[mr-c]

There is a first stab at this in #704