src/main.c: Add error message when no command could be run
src/main.c: Add error message when no command could be run

All commands need a target to operate on. Running a command with no target would
produce no output which can be confusing. Instead return an error message and a
hint.

Signed-off-by: Alistair Popple <e9adbcf81c8a36ccc628e09bc92d0e25e1c8e501@popple.id.au>

open-power/pdbg,open-power/pdbg,open-power/pdbg