Pass reader error messages along (#28)

This fixes problems like the one mentioned in issue #27 where using the
reader in Python 2.x with it's default `str` type would throw an error,
but the error message was overridden. There was a similar change made
for issue #5.

I don't believe a new test is required for this since
https://github.com/ryanhiebert/backports.csv/blob/1.0.5/tests.py#L853-L856
covers the source exception and this is more about the messaging
bubbling up. It would be possible if needed to check the message higher
up the chain though.

Author: mpeteuil

src/backports/csv.py

@@ -228,8 +228,8 @@ def __init__(self, fileobj, dialect='excel', **fmtparams):
 
         try:
             self.dialect = Dialect.combine(dialect, fmtparams)
-        except Error:
-            raise TypeError('Invalid dialect parameters')
+        except Error as e:
+            raise TypeError(*e.args)
 
         self.fields = None
         self.field = None