obi grep: made an error message clearer (error could be eventually be

handled by program, looking for str in bytes returned by a column)

python/obitools3/commands/grep.pyx

@@ -207,9 +207,12 @@ def Filter_generator(options, tax_filter):
                    )
              
         if good and predicates:
+            try:
                 good = (reduce(lambda bint x, bint y: x and y,
                         (bool(eval(p, loc_env, line))
                         for p in predicates), True))
+            except TypeError:
+                raise Exception("Try replacing strings in python expression with bytes (by putting 'b' in front of strings, e.g. b'example')")
 
         if good and lmin:
             good = len(line) >= lmin