refactor-864370: Update error outputting

V8 is weird about logging exceptions, so we group all the logic about
getting a sane stack trace on all platforms together in one place.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
refactor-864370: Test harness updates

The 'sample test' output had the wrong label for the error message and
we were not outputting the type and error properties for executed output

In mockCommands we export the commands rather than the command specs.
This makes writing tests that compare values with actual commands
easier.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
remotegcli-839862: Update 'context' command

Allow a defaultValue of null to enable people to reset the context.
This means adding a new output string.

Signed-off-by: Joe Walker <jwalker@mozilla.com>
remotegcli-839862: Add tests for 'context' command

Signed-off-by: Joe Walker <jwalker@mozilla.com>