Schematics template syntax errors should point to the offending template

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x] feature request
```

### Repro steps

- Have a syntax error in your template.
- Run the schematic with `--debug`, and get a stack trace that points to schematics code, but that doesn't point to any of your own code.

### The log given by the failure
This is easier to handle when you when the syntax error is something simple like misspelling a variable name, e.g. you pass in template variable `foobar` and have a template with `<%= foobat %>`:
```
An error occured:
ReferenceError: foobat is not defined
at eval (eval at template (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/node_modules/@angular-devkit/core/src/utils/template.js:257:16), :22:18)
at /usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/template.js:35:79
at tree.visit (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/base.js:130:30)
at files.forEach.path (/usr/local/google/home/nsidhom/.nvm/versions/node/v10.1.0/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/src/tree/virtual.js:131:40)
at Array.forEach ()
at FileSystemCreateTree.visit (/usr/local/google/home/nsidhom/.nvm/versions/node/v10.1.0/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/src/tree/virtual.js:131:24)
at /usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/base.js:126:14
at MergeMapSubscriber.input.pipe.operators_1.mergeMap.inputTree [as project] (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/call.js:77:24)
at MergeMapSubscriber._tryNext (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/node_modules/rxjs/internal/operators/mergeMap.js:65:27)
at MergeMapSubscriber._next (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/node_modules/rxjs/internal/operators/mergeMap.js:55:18)
```

But if you have a syntax error such as putting an unintended second symbol within the brackets like `<%= foobar unintended %>`, then you just get `SyntaxError: Unexpected identifier`, which could be anywhere and is hard to hunt down:
```
An error occured:
SyntaxError: Unexpected identifier
at Function ()
at Object.template (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/node_modules/@angular-devkit/core/src/utils/template.js:257:16)
at /usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/template.js:35:40
at tree.visit (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/base.js:130:30)
at files.forEach.path (/usr/local/google/home/nsidhom/.nvm/versions/node/v10.1.0/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/src/tree/virtual.js:131:40)
at Array.forEach ()
at FileSystemCreateTree.visit (/usr/local/google/home/nsidhom/.nvm/versions/node/v10.1.0/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/src/tree/virtual.js:131:24)
at /usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/base.js:126:14
at MergeMapSubscriber.input.pipe.operators_1.mergeMap.inputTree [as project] (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/src/rules/call.js:77:24)
at MergeMapSubscriber._tryNext (/usr/local/google/home/nsidhom/schematics/pantheon/node_modules/@angular-devkit/schematics/node_modules/rxjs/internal/operators/mergeMap.js:65:27)
```

### Desired functionality
I would like for the error to point me to the offending template, and if possible, the offending line of the template. As is, these errors are incredibly hard to debug.

Reproduziere den fehlerhaft formatierten Template-Ausdruck mit schematics, ausgeführt mit --debug, und beginne dann bei den im Stacktrace angezeigten template.js-Stellen, insbesondere @angular-devkit/core/src/utils/template.js und @angular-devkit/schematics/src/rules/template.js. Als erledigt gilt die Aufgabe, wenn Syntaxfehler das fehlerhafte Template und, sofern möglich, seine Zeile identifizieren, statt nur eine generische Parserposition zu melden.

Vom Indexierungsmodell aus dem Issue-Text verfasst.