Useful error message when HelpMessage attributes are empty string

[SteveL-MSFT]

Fix invalid value error message on some cmdlet attributes to point to the specific attribute.

Removed wrapping of argument exception into not useful requires exception

Before:

.\helpmessage.ps1 : Cannot process the #requires statement because it is not in the correct format.
The #requires statement must be in one of the following formats:
 "#requires -shellid <shellID>"
 "#requires -version <major.minor>"
 "#requires -psedition <edition>"
 "#requires -pssnapin <psSnapInName> [-version <major.minor>]"
 "#requires -modules <ModuleSpecification>"
 "#requires -runasadministrator"
At line:1 char:1
+ .\helpmessage.ps1
+ ~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (helpmessage.ps1:String) [], CommandNotFoundException
    + FullyQualifiedErrorId : ScriptRequiresInvalidFormat

After:

Cannot process argument because the value of argument "HelpMessage" is not valid. Change the value of the
"HelpMessage" argument and run the operation again.
At line:1 char:1
+ .\helpmessage.ps1
+ ~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidArgument: (:) [], RuntimeException
    + FullyQualifiedErrorId : Argument

Fix #4331

[lzybkr]

I sign off assuming the #requires error message has not degraded from this change.

[SteveL-MSFT]

@lzybkr ScriptRequiresException still gets thrown in all other cases