Updated PSLIB_InvalidPattern0 to be more readable

[abatishchev]

The only place this localized string used is here:

azure-pipelines-task-lib/powershell/VstsTaskSdk/LegacyFindFunctions.ps1

Lines 134 to 137 in 5adadae

	# Validate pattern does not end with a \.
	if ($pattern[$pattern.Length - 1] -eq [System.IO.Path]::DirectorySeparatorChar) {
	throw (Get-LocString -Key PSLIB_InvalidPattern0 -ArgumentList $pattern)
	}

In real world, the error looks like this

##[error]Invalid pattern: 'C:\__w\1\s\src\Application\pkg\Release\'

Which is too vague to make it possible to troubleshoot without finding and looking into the source code. I ran into this error here: microsoft/azure-pipelines-tasks#17106.

This PR includes:

1. improving the error message.
2. using EndsWith() instead of $str[$str,Legth -1] for readability

However, what doesn't include:

1. I didn't rename the placeholder
2. nor updated the localizations

Please let me know if I should do the former and what's the process for the latter (I can update the ru-ru string though).

[kirill-ivlev]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[abatishchev]

Awesome, thanks everyone! Can someone who has the permissions to complete the PR?

[localconst]

Done, thanks for the contribution!