Change error message casing

[e-dard]

When updating to v3, one of our unit tests broke. Looking into it, I see at some point error messages were capitalised.

Error messages should not be capitalised unless they're acronyms or proper nouns, so that they can be composed with other text when, for example, logging them.

See here for more details.

[dgrijalva]

Thanks for catching this. Let me review the history of these strings and think about how this semver applies here.

The reason those errors are published is so you can compare against them directly. Is there a reason you don't use the constants in your tests directly?

[e-dard]

The one that caught us out was the expiration one, which does not have an exported variable. I'm happy to add explicit variables in this PR, which would be a further improvement.