Better error message when missing args to pdb command

Current behaviour:
```
λ ./python.exe -c 'breakpoint()'
> <string>(1)<module>()
-> breakpoint()
(Pdb) source
*** SyntaxError: invalid syntax
```

Proposed behaviour:
```
./python.exe -c 'breakpoint()'
> <string>(1)<module>()
-> breakpoint()
(Pdb) source
*** Missing arg!
```


### Linked PRs
* gh-126945
* gh-130240