Include snapshot name in failure message, if given

[calda]

I'm setting up snapshot testing for the Lottie project (airbnb/lottie-ios#1432). We're currently running snapshot tests using this library via the command line in CI.

Our tests are setup dynamically, so all run in the same testLottieSnapshots method but have a different name (the name of the file being snapshot).

Currently, the assertion messages only include the name of the test case. This makes it challenging to know which individual snapshot is failing:

    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot does not match reference.

This PR updates the assertion failure message to include the snapshot name, if given. With this change, it's much easier to tell which individual snapshot is failing:

    ✖ testLottieSnapshots, failed - Snapshot "LottieLogo1 (0%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "LottieLogo1 (50%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "LottieLogo1 (100%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "Apostrophe (0%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "Apostrophe (50%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "Apostrophe (100%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "P (0%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "P (50%)" does not match reference.
    ✖ testLottieSnapshots, failed - Snapshot "P (100%)" does not match reference.

[mbrandonw]

Looks great @calda, thanks for the PR!