Improve error message thrown when serializable object is not found

[caisq]

in name-class map

Fixes tensorflow/tfjs#625

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This change is 

[dsmilkov]

Thank you!

Reviewed 1 of 1 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @caisq, @ericdnielsen, @dsmilkov, and @bileschi)

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src/utils/generic_utils.ts, line 178 at r1 (raw file):

            `2. The custom ${printableModuleName} is defined in JavaScript, ` +
            `but is not registered properly with ` +
            `tf.serialization.registerClass().`);

Would be nice to point to the custom layers example: https://github.com/tensorflow/tfjs-examples/tree/master/custom-layer

[caisq]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @caisq, @ericdnielsen, and @bileschi)

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src/utils/generic_utils.ts, line 178 at r1 (raw file):

Previously, dsmilkov (Daniel Smilkov) wrote…

Would be nice to point to the custom layers example: https://github.com/tensorflow/tfjs-examples/tree/master/custom-layer

Thanks for the suggestion. I was thinking of the same thing. However, I'd prefer the link to be a tutorial or how-to page on js.tensorflow.org, which looks more permanent and well-organized. We don't have such a page yet, but I'm planning to add one. I added a TODO item here.