Exception is thrown if repository folder doesn't exist

[fabasoad]

Describe the bug
A clear and concise description of what the bug is.

Steps to Reproduce

1. Run ghacu -r "not-existing-folder"

Expected behavior
Message should be shown, like "not-existing-folder" does not exist.

Actual behavior
Exception is thrown:

C:\Users\vanil>ghacu -r "C:\Users\vanil\Documents\test" -u
Unhandled exception. Ghacu.Workflow.Exceptions.WorkflowValidationException: Directory C:\Users\vanil\Documents\test does not exist.
   at Ghacu.Workflow.WorkflowService.GetWorkflowPath(String repositoryPath) in /Users/10593/Source/personal/ghacu/src/Ghacu.Workflow/WorkflowService.cs:line 42
   at Ghacu.Workflow.WorkflowService.GetWorkflows(String repositoryPath) in /Users/10593/Source/personal/ghacu/src/Ghacu.Workflow/WorkflowService.cs:line 23
   at Ghacu.Runner.Cli.CliService.Run(String repository, Boolean shouldUpgrade) in /Users/10593/Source/personal/ghacu/src/GHACU/Cli/CliService.cs:line 32
   at Ghacu.Runner.Program.<>c.<Main>b__0_0(Options o) in /Users/10593/Source/personal/ghacu/src/GHACU/Program.cs:line 62
   at CommandLine.ParserResultExtensions.WithParsed[T](ParserResult`1 result, Action`1 action)
   at Ghacu.Runner.Program.Main(String[] args) in /Users/10593/Source/personal/ghacu/src/GHACU/Program.cs:line 63

Screenshots
N/A

Technical information (please complete the following information):

• OS: Windows 10
• ghacu version 2.0.0
• Command line name - cmd

Additional context
N/A