Ensure sandbox/aztec.js errors are clear

[PhilWindle]

Look for obvious errors that will be made by developers (forgetting to add an account/public key for example) and ensure we give them as much help as possible.

Ensure errors are propagated out to aztec.js.

Tasks

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1. Ensure AztecRpc server never returns outdated info #1222
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2. Throw an error if contract doesn't exist in a contract tree when creating an instance #1225
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3. Create a type with validity checks for private key #1226
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4. Add runtime check of input's validity to encoder #1227
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5. Throw an error when requesting public storage for non-existent contract #1228
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6. Rename stev function to compute_note_hash_and_nullifier #1229
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7. Make send method check whether origin address is registered #1230
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8. Rename 'take' to 'limit' #1231
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9. Alert when create_hash_and_nullifier function is missing #1303
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[spalladino]

Sync with @rahul-kothari on the error codes page for circuits errors, we may be able to reuse efforts from there

[rahul-kothari]

Ah thanks for adding me to this. @PhilWindle I added some common circuit errors here - https://github.com/AztecProtocol/aztec-packages/blob/master/docs/docs/aztec/protocol/errors.md

includes stuff like "you have too many read requests/created too many commitments...."

[benesjan]

Tracking error issues in this file. I am addressing issues which are easy to fix straight away in this PR.

Update: Santiago agreed with addressing the issue in the file and I've created separate issues for all of those and linked them as tasks in the description of this issue.

[PhilWindle]

Closing this issue. The outstanding tasks are quite involved so I don't think we should rely on them being completed to release the sandbox.