Improve errors and docs for `mv` command [Good first issue]

[ficocelliguy]

Currently, the mv command fails wit an unhelpful error if the script that was (attempted to be) renamed is currently running.

The error should be more descriptive, indicating that the script cannot be renamed/moved because it is currently running.

The documentation on mv should also mention this restriction.

context:
https://discord.com/channels/415207508303544321/415207508303544323/1374815018121691136

[d0sboots]

We could also simply remove this restriction; Linux/unix doesn't have it, and there's no technical reason we need to have it either.

[UncleCeiling]

Following a discussion on Discord I'll make the changes to the error message for both the command and API call.

Changing the behaviour of mv/rm would result in some funkiness with the WorkerScript behaviour on load that I'm not capable of handling yet.

Error: WorkerScript constructed with invalid script filename: looper.js
    at new WorkerScript (webpack-internal:///./src/Netscript/WorkerScript.ts:88:13)
    at createAndAddWorkerScript (webpack-internal:///./src/NetscriptWorker.ts:140:24)
    at startWorkerScript (webpack-internal:///./src/NetscriptWorker.ts:96:7)
    at loadAllRunningScripts (webpack-internal:///./src/NetscriptWorker.ts:239:7)
    at Object.load (webpack-internal:///./src/engine.tsx:280:79)

[d0sboots]

Yes, that's a good point. Scripts currently assume they can be restarted on load; technically this could mean a different version of the code if they were changed, but we're usually fine with that. It gets weirder if you remove the code, since that breaks.