Even better errors for incorrect usage

Split file not found from wrong extension; was still a bit unclear.

src/CLI.cs

@@ -49,15 +49,18 @@ public int Run(string[] args)
 		} catch(InvalidSlnPathException ex) {
 			Console.Error.WriteLine(ex.Message);
 			return 2;
-		} catch(PathNotFoundException ex) {
+		} catch(SlnFileNotFoundException ex) {
 			Console.Error.WriteLine(ex.Message);
 			return 3;
+		} catch(PathNotFoundException ex) {
+			Console.Error.WriteLine(ex.Message);
+			return 4;
 		} catch(EmptyPathListException) {
 			Console.Error.WriteLine("No paths supplied to sync with.");
 			Console.Error.WriteLine("Supply a list of folder/file arguments that you want to be sync'd into the target solution folder.");
 			Console.Error.WriteLine();
 			WriteUsage();
-			return 4;
+			return 5;
 		}
 
 		Console.Out.WriteLine("Sync completed.");

src/SlnSync.cs

@@ -20,9 +20,13 @@ public SlnSync() : this(new DefaultGuidGenerator())
 	/// <param name="paths">list of paths to recursively add/update SolutionItems virtual folders with</param>
 	public void SyncSlnFile(string slnPath, string slnFolder, IEnumerable<string> paths)
 	{
-		if (!File.Exists(slnPath) || !slnPath.EndsWith(".sln"))
+		if (!slnPath.EndsWith(".sln"))
 		{
-			throw new InvalidSlnPathException($"'{slnPath}' is not a sln file");
+			throw new InvalidSlnPathException(slnPath);
+		}
+		if (!File.Exists(slnPath))
+		{
+			throw new SlnFileNotFoundException(slnPath);
 		}
 		if (!paths.Any())
 		{
@@ -121,6 +125,7 @@ private SolutionFolder FindOrCreateSolutionFolder(ICollection<IProject> solution
 		=> solutionProjects.OfType<SolutionFolder>().FirstOrDefault(project => project.Name == folderName);
 }
 
-public class PathNotFoundException(string path) : Exception($"Path '{path}' not found");
-public class InvalidSlnPathException(string s) : Exception(s);
+public class PathNotFoundException(string path) : Exception($"Path not found: '{path}'");
+public class SlnFileNotFoundException(string path) : Exception($"Invalid .sln file '{path}' - File not found.");
+public class InvalidSlnPathException(string path) : Exception($"Invalid .sln file '{path}' - File must have .sln extension");
 public class EmptyPathListException() : Exception();