Warn about *[i18n-content] elements having child elements (not supported).

Pushing HTML content markup to chrome.i18n message files would be a bad idea.

Add warning to applyChromeI18nMessages.js when chrome.i18n is not available, but tolerate it (using chrome extension pages stand-alone for content  development).
Improve text content restriction warning.
Clarify reason for use of innerText.
Improve wording of a warning.

Untangle from issue35 with "git rebase -i 67ecc84"