Make MantaClientHttpResponseException extend IOException for better back...

...wards compatibility.

Previously, where clients didn't need access to specific HTTP fields contained in HttpResponseException, they have been catching HttpResponseException as IOException (because former is a subclass of latter), for example in recursive delete logic:
```
try { 
    client.delete(blah);
} catch (IOException e) {
    client.deleteRecursively(blah);
} 
```

Now that we switched to MantaClientHttpResponseException, this is no longer possible, and two exceptions need to be handled in the same way:
```
boolean deleteRecursively = false;
try { 
    client.delete(blah);
} catch (IOException e) {
   deleteRecursively = true;
} catch (MantaClientHttpResponseException e) {
   deleteRecursively = true;
}
if (deleteRecursively) {
    client.deleteRecursively(blah);
}
```
However, the old logic may be achieved if MantaClientHttpResponseException extends IOException instead of MantaClientException.