Error message when compilation fails

Within selection, the compilation of the string into python bytecode
can fail.

This shows an appropriate message.

relational/relation.py

@@ -130,7 +130,10 @@ class Relation (object):
         newt = relation()
         newt.header = Header(self.header)
 
+        try:
             c_expr = compile(expr, 'selection', 'eval')
+        except:
+            raise Exception('Failed to compile expression: %s' % expr)
 
         for i in self.content:
             # Fills the attributes dictionary with the values of the tuple