SORU: Improve error message for missing key for rollup client

Context

Improve error message for missing key in json parsing. Currently, there is no mentioning of which key is missing when, for instance, passing an incorrect json value for proof generation.

Manually testing the MR

Test that the following command:

> ./octez-smart-rollup-client-alpha get proof for message 0 of outbox at level 2297 transferring "{\"foo\": \"bar\"}"

Results in an error:

Error
  Invalid_argument("Key \"destination\" was not found in: {\"foo\":\"bar\"}: ")

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR