feat(parser): TemplateParser.tryParse() returns both the AST and errors

[chuckjaz]

• Please check if the PR fulfills these requirements
• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Adds tryParse() to TemplateParser that always returns an AST even if there are errors.

• What is the current behavior? (You can also link to an open issue here)

TemplateParser has a method parse() that throws when the template has errors.

• What is the new behavior (if this is a feature change)?

TemplateParser as a method tryParse() that always returns an AST even if there are errors.

The language service (#7482) needs the AST even if there are errors.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

[mary-poppins]

Merging PR #7858 on behalf of @mhevery to branch presubmit-mhevery-pr-7858.

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