Remove explicit multiline error string concatenation

There are a few places that had strings broken onto multiple lines using string interpolation AND concatenation.  This results in errors like the one below:

```
Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05) 
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ipcalc
>>> ipcalc.Network("256.0.0.0")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/jwineinger/dev/8b/lib/python2.7/site-packages/ipcalc.py", line 166, in __init__
    self.ip = self._dqtoi(ip)
  File "/Users/jwineinger/dev/8b/lib/python2.7/site-packages/ipcalc.py", line 313, in _dqtoi
    'bytes should be between 0 and 255' % dq)
TypeError: not all arguments converted during string formatting
```

I simply removed the concat operator -- the "+" -- so that python implicitly joins the lines together and then applies the string formatting.