Silent fail when GDB is not installed

[pvanderlinden]

When gdb is not installed, pyrasite will not give an error, warning or message, but it won't do anything as well. It would be good to have this as a message like when ptrace is disabled.

[gw0]

Yes, this would be great. I had to add verbose=True into inject() function (as mentioned in #60) to figure that out:

# pyrasite-shell 25657

/bin/sh: 1: gdb: not found