Fixup strong name signing (#15266)

Author: mmitche

src/Microsoft.DotNet.SignTool/src/RealSignTool.cs

@@ -155,7 +155,10 @@ public override bool LocalStrongNameSign(IBuildEngine buildEngine, int round, IE
             {
                 if (file.SignInfo.ShouldLocallyStrongNameSign)
                 {
-                    return LocalStrongNameSign(file);
+                    if (!LocalStrongNameSign(file))
+                    {
+                        return false;
+                    }
                 }
             }
 

src/Microsoft.DotNet.SignTool/src/ValidationOnlySignTool.cs

@@ -38,7 +38,10 @@ public override bool LocalStrongNameSign(IBuildEngine buildEngine, int round, IE
             {
                 if (file.SignInfo.ShouldLocallyStrongNameSign)
                 {
-                    return LocalStrongNameSign(file);
+                    if (!LocalStrongNameSign(file))
+                    {
+                        return false;
+                    }
                 }
             }