Avoid try/catch for 'DarkYellow'

[dahlbyk]

Error log in #771 (comment) pointed out using 'DarkYellow' ends up polluting $Errors with:

Exception calling "FromHtml" with "1" argument(s): "DarkYellow is not a valid value for Int32."
At C:\Dev\OSS\posh-git\src\AnsiUtils.ps1:70 char:17
+                 $color = $ColorTranslatorType::FromHtml($color)

Since DarkYellow is the only [ConsoleColor] that's not also represented in HTML, a hard-coded exception seems appropriate.

cc #536

[rkeithhill]

Minor knits otherwise LGTM.

[dahlbyk]

Minor knits otherwise LGTM.

Better?

[rkeithhill]

Yup. LGTM.