error out in some form when no table could be found parsing an elf for defmt

[Vollbrecht]

current behavior just silently does nothing if no table found when user runs espflash like espflash monitor --elf=test --log-format=defmt

offending part https://github.com/esp-rs/espflash/blob/main/espflash/src/cli/monitor/parser/esp_defmt.rs#L88

[bugadani]

defmt also doesn't make sense if there is no elf file specified (or inferred) anyway. So we just shouldn't proceed if the result of load_table is None, and we should probably replace the Option with a Result and meaningful errors.