lang: Fix missing error messages of the generated errors in declare_program!

[acheroncrypto]

Problem

declare_program! doesn't account for custom error messages during error generation. For example, with the following error definition:

#[error_code]
pub enum ExternalError {
    Default,
    #[msg("Custom msg")]
    WithMsg,
}

and the following assertions:

assert_eq!(ExternalError::Default.to_string().as_str(), "Default");
assert_eq!(ExternalError::WithMsg.to_string().as_str(), "Custom msg");

the last assertion fails:

thread 'test_error_msg' (406948) panicked at programs/declare-program/tests/error.rs:19:5:
assertion `left == right` failed
  left: "WithMsg"
 right: "Custom msg"

Summary of changes

• Add the missing error messages to the generated errors in declare_program!
• Add error message generation tests and move all error tests to a dedicated file

[vercel]

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