pysh: Say what kind of token isn't implemented

When the shell lexer finds an unrecognized dollar token, the error
message should contain what kind of token it is having problems with.

Signed-off-by: Olof Johansson <olof.johansson@axis.com>
initial work on supporting shell arith

Signed-off-by: Olof Johansson <olof.johansson@axis.com>