fix(@angular-devkit/schematics): include the template path in errors thrown by content templates - #34621

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular-cli/blob/main/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Errors thrown while applyContentTemplate renders a file don't say which file. With a misspelled variable, ng generate prints only foobat is not defined. A syntax error inside <% %> prints Unexpected identifier 'unintended'. In both cases the stack points only into @angular-devkit/core.

There is a related bug in the same try block. Since 18.1.0 (41ee8ea), the catch returns the entry unchanged for any TypeError, so that content that isn't valid UTF-8 is left alone. Because decoding and rendering share that try, a TypeError thrown by the template itself is also ignored, for example <%= name.length %> with name undefined. The schematic then succeeds and writes the unrendered template.

Issue Number: #13192

What is the new behavior?

The PR has two commits.

1. Decode and render separately. Content that can't be decoded is still returned unchanged. A TypeError from the template is now reported, as it was before 18.1.0.
2. Name the template. An error from rendering is rethrown with the template path, keeping the original error as cause, like HostTree's Failed to decode "<path>" error. With a template at files/sub/__name__.ts.template applied through url('./files'), the message is:

Failed to apply template "/sub/__name__.ts.template": ReferenceError: foobat is not defined

Does this PR introduce a breaking change?

• Yes
• No

Other information

Tests. The specs in template_spec.ts cover:

• a TypeError thrown by a template;
• content that isn't valid UTF-8, which is still returned unchanged;
• a ReferenceError and a SyntaxError, checking the message and the cause;
• an applyTemplates case that checks the path in the message.

I ran them with jasmine and tsx, outside Bazel. Each commit's new specs fail without its change and pass with it.

Other suites. All 141 specs under packages/angular_devkit/schematics pass, except two FileSystemEngineHost specs that need a Bazel-built fixture; they fail the same way on main.

Built-in templates. Because template TypeErrors used to be ignored, I also ran the 930 @schematics/angular specs on main and on this branch. I generated the schema .ts files with tools/quicktype_runner.js first. The same 18 specs fail on both in this Windows setup, so none of the built-in templates relied on the old behaviour.

Checks. tsc 6.0.3 and prettier are clean for the changed files.

Side effects. A SchematicsException thrown on purpose by a template helper is now wrapped the same way. Its message still appears after the path, and the exception itself is kept as cause.

Not covered. The message doesn't include the line inside the template, because the template is compiled with Function() and its line numbers don't match the template's.