Show fabfile line error

What do you think to this feature.

Before : 

```
[localhost] local: tar -czf /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh/shorewall.tar.gz -C assets shorewall
[127.0.0.1] put: /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh/shorewall.tar.gz -> /etc/shorewall.tar.gz
[127.0.0.1] run: tar -xzf shorewall.tar.gz
[127.0.0.1] run: rm -f shorewall.tar.gz
[localhost] local: rm -rf /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh
[127.0.0.1] run: shorewall start
[127.0.0.1] out: Compiling...
[127.0.0.1] out: ERROR: /etc/shorewall.conf does not exist!
[127.0.0.1] out: 
Fatal error: run() received nonzero return code 2 while executing!
 
Requested: shorewall start
Executed: /bin/bash -l -c "shorewall start"

 
Aborting.
Disconnecting from 127.0.0.1:2200... done.
```

after

```
[localhost] local: tar -czf /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh/shorewall.tar.gz -C assets shorewall
[127.0.0.1] put: /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh/shorewall.tar.gz -> /etc/shorewall.tar.gz
[127.0.0.1] run: tar -xzf shorewall.tar.gz
[127.0.0.1] run: rm -f shorewall.tar.gz
[localhost] local: rm -rf /var/folders/7v/bs1393w960b4_g1_2b5h9sf00000gn/T/tmpAu4Ueh
[127.0.0.1] run: shorewall start
[127.0.0.1] out: Compiling...
[127.0.0.1] out: ERROR: /etc/shorewall.conf does not exist!
[127.0.0.1] out:
 
 
Fatal error: run() received nonzero return code 2 while executing!
 
Requested: shorewall start
Executed: /bin/bash -l -c "shorewall start"

Error at: /Users/stephane/myproject/fabfile.py:136 - install_shorewall()
 
Aborting.
Disconnecting from 127.0.0.1:2200... done.
```
Now I can see that my error is at line 136 in my fabfile.py, and the python code is « install_shorewall() ».

What do you think about this suggestion to help debug process ?