Implement 'ErrorMessage' attribute property when this PowerShell feature is released

[marckassay]

When the following PowerShell build has been released (for some time?):
fa901b646f9b194498c5d94456c2bdd7dab05680

Have this feature added to New-PSTrueCryptContainer's 'MountLetter' attribute:

[Parameter(Mandatory = $True, Position = 3)]
[ValidatePattern("^[a-zA-Z]$", ErrorMessage="New-PSTrueCryptContainer : Cannot validate argument on parameter 'MountLetter'. The argument '{0}' does not match.  Try again using exactly one character from the 'A-Z' set.")]
[string]$MountLetter,