[FEATURE] Add a more actionable error when user attempts to `bb verify` without existing vk

[Savio-Sou]

Problem Statement

In occasions like #17663, bb users who:

1. bb prove a Noir program without --write_vk
2. Then attempts to bb verify such Noir program

Would be prompted with a somewhat confusing error:

Unable to open file: ./target/vk

Proposed Solution

Extend the error message for when vk is not found to e.g.:

Unable to find file: ./target/vk.
Generate a vk during proving by running bb prove with an additional --write_vk flag.

Example Use Case

No response

Alternative Solutions

No response

Additional Context

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