Add custom error message for StorageNoopGuard

[seadanda]

Expand StorageNoopGuard to be able to add extra context through a custom error message. When the guard is triggered it panics with an error message which can be defaulted, set on construction, or set after it has been constructed.

Turn StorageNoopGuard into struct with storage_root and error_message and added from_error_message constructor and set_error_message setter.

Also added new() aliased to default().

Closes #375

[liamaharon]

Looking good, left some comments.

Also, I think we should also add something like

impl From<&'static str> for StorageNoopGuard {
    fn from(s: &'static str) -> Self {
        StorageNoopGuard {
            storage_root: sp_std::vec::Vec::new(),
            error_message: s,
        }
    }
}

[seadanda]

Looking good, left some comments.

Also, I think we should also add something like

impl From<&'static str> for StorageNoopGuard {
    fn from(s: &'static str) -> Self {
        StorageNoopGuard {
            storage_root: sp_std::vec::Vec::new(),
            error_message: s,
        }
    }
}

Great, seems like a nice addition. I'll add it in.

[liamaharon]

LGTM once impl From<&'static str> for StorageNoopGuard is added

.

[seadanda]

OK to summarise for all reviewers:

• I have made all suggested improvements to the doc comments and error messages (and thus updated the tests)
• In the end I've removed the From implementation as it is very close to from_error_message and @ggwpez pointed out that conceptually it is different from how From is normally used.
• I've switched the type to String now that I understand that it is purely for testing and I don't need to unnecessarily optimise, especially with the benefits that String brings.

I hope everybody is happy with the changes I've gone with. Thanks for the very helpful reviews :)

[paritytech-cicd-pr]

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