Need Graceful Response if executing PlatyPS commands from non-FileSystem provider

[jongeller]

discovered this when trying to run PlatyPS on the SQLPS module for SQL server. Part of getting the commands moves the runtime over to the SQL provider, PlatyPS fails dramatically at this point.

To Reproduce

Set-Location HKLM: #this puts you on a provider outside of the filesystem (registry)
New-ExternalHelp -Path <MdFilePath> -Output <outputPath>

Expected behavior

You get the help.xml (external help) expected.

Actual behavior

Get-MarkdownMetadata : Cannot bind argument to parameter 'Markdown' because it is an empty string.
At D:\GitHub Repos\platyPS\out\platyPS\platyPS.psm1:1155 char:48
+     $metadata = Get-MarkdownMetadata -markdown $markdown
+                                                ~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Get-MarkdownMetadata], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorEmptyStringNotAllowed,Get-MarkdownMetadata

Environment data

PlatyPS v0.5.0
Windows 10
PowerShell 5.0

[vors]

That blocks SQLPS help migration, because you can only run these command from the SQL provider.

[vors]

This is actually applicable to New-ExternalHelp

[jongeller]

resolved in e07fd67 - waiting for build to pass, then will merge into master along with other minor fixes.

[jongeller]

fixed tests in 0a5c56b will merge in to master on build test completion.

[jongeller]

one build error remains. working on it now. it does not exist locally.

[jongeller]

This issue is now also a fix, if the file system provider in not in use. PlatyPS will change providers to the first available file system. This means C: will typically be the preferred drive. 6db781e