Add an error message when PC is invalid
Add an error message when PC is invalid

Closes #160

zachriggle/pwndbg,0xddaa/pwndbg,cebrusfs/217gdb,disconnect3d/pwndbg,pwndbg/pwndbg,anthraxx/pwndbg,disconnect3d/pwndbg,0xddaa/pwndbg,pwndbg/pwndbg,zachriggle/pwndbg,anthraxx/pwndbg,cebrusfs/217gdb,pwndbg/pwndbg,0xddaa/pwndbg,anthraxx/pwndbg,chubbymaggie/pwndbg,cebrusfs/217gdb,anthraxx/pwndbg,pwndbg/pwndbg,cebrusfs/217gdb,disconnect3d/pwndbg,chubbymaggie/pwndbg