Handle error in Get-DOTExecutable.ps1

[t-mxcom]

Hello!

Line 9 in Get-DOTExecutable.ps1 causes an error and terminates the execution of AzViz, if the dot application wasn't found in the system search path.
This should be handled so that it's not necessary to add Graphviz to $env:path if it's installed in one of the predefined default directories (lines 4-7).

I'm currently using PowerShell version

Major	Minor	Build	Revision
5	1	19041	1320

Thank you!