correction for line=None [ticket:25]

lib/mako/exceptions.py

diff --git a/lib/mako/exceptions.py b/lib/mako/exceptions.py
index 9994ace..264068e 100644
--- a/lib/mako/exceptions.py
+++ b/lib/mako/exceptions.py
@@ -155,7 +155,7 @@
 Traceback (most recent call last):
 % for (filename, lineno, function, line) in tback.traceback:
   File "${filename}", line ${lineno}, in ${function or '?'}
-    ${line.strip()}
+    ${line | unicode.strip}
 % endfor
 ${str(tback.error.__class__.__name__)}: ${str(tback.error)}
 """)
@@ -220,4 +220,4 @@
 
 </body>
 </html>
-""")
\ No newline at end of file
+""")