Add error display to core widgets.

[dkbennett]

Summary of the pull request

Core widgets currently do not set ContentData to anything on error, which causes them to display an empty template with variable names with no indication of what is wrong. This change adds an error message to the main template for each of the core widgets, and populates that message with any exception errors when encountered so it is obvious to the user that something is wrong. If there is no error message, then the templates display as normal, but if there is an error message that is displayed instead.
 

References and relevant issues

Closes #1248

Detailed description of the pull request / Additional comments

• Each core widget now sets an error message when an exception occurs during LoadContentData.
• Each core widget template has a container for displaying any error messages that is mutually exclusive with its normal display (if error, show error, else show normal template).

The diff of the templates looks like a lot changed, but the actual change was adding a new container for the error message and then shifting the previous content to be in a second container mutually exclusive with the error container. Shifting it one indentation level made for a messy JSON diff.

Validation steps performed

• Manual test by forcing failure in the LoadContentData method of each widget.
• Confirmed that the widgets look normal without failure and that they show the failure message with
• Exception: I was unable to validate the SSH widget as I had no SSH config file.

PR checklist

• Closes #xxx
• Tests added/passed
• Documentation updated

[AmelBawa-msft]

Consider promoting error message object creation to a method in the parent class (reusability + maintenance)