fix(codewhisperer): update security scan error message (#2770)

## Problem
If customer opens a non-text document, for example images, and hit security scan, they will see Please open a file you want to scan to proceed, which can be confusing.

## Solution
To update the error message to handle this case.

Author: zixlin7

src/codewhisperer/commands/basicCommands.ts

@@ -158,7 +158,7 @@ export const showSecurityScan = Commands.declare(
                     startSecurityScanWithProgress(securityPanelViewProvider, editor, client, context.extensionContext)
                 }
             } else {
-                vscode.window.showInformationMessage('Please open a file you want to scan to proceed.')
+                vscode.window.showInformationMessage('Please open a valid file you want to scan to proceed.')
             }
         }
 )