Fix/fix assert.true

[ubyjvovk]

In generated assert-style checks, assert.true was used, which gives error assert.true is not a function. This PR changes all assert.true to simply assert.

Also, this adds a simple message to failed schema validation asserts

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[ubyjvovk]

@googlebot I signed it!

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We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for all the commit author(s) or Co-authors. If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.
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