Improve templating error reporting

[lbussell]

Our previous error reporting only reported the file that template errors are in and then dumped some of the template text out.

This PR improves it by actually printing which line and column the errors are on. The error location format is {startLine},{startColumn},{endLine},{endColumn}, in order to comply with VS Code's problem matching syntax.

With a custom template generation task and problem matcher, this allows us to get accurate error reporting for template generation:

{
    "label": "Generate Dockerfiles",
    "type": "shell",
    "command": "pwsh ./eng/dockerfile-templates/Get-GeneratedDockerfiles.ps1",
    "group": "build",
    "problemMatcher": [
        {
            "owner": "generate-dockerfiles",
            "fileLocation": [
                "relative",
                "${workspaceFolder}"
            ],
            "pattern": [
                {
                    "regexp": "^(?: )*Template parsing error in file (.*):(.*)(?: )*$",
                    "file": 1,
                    "location": 2
                },
                {
                    "regexp": "^(?: )*Message: (.*)$",
                    "message": 1
                }
            ]
        }
    ]
},

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