improve error message for missing shell expression

Summary:
I had the following Markdown test:

```
# Make sure Scrut metadata is included.

## Test

\`\`\`scrut
DEBUG_PAR_MAIN="yes" $CLI 2>&1
[0]
\`\`\`
```

I forgot to prefix it with $. This adds a suggestion to the error so it's more obvious.

Reviewed By: abesto

Differential Revision: D61016388

fbshipit-source-id: da8d7fe9c4dae21d8636fd0e75c0dd4cdb9eb027

src/parsers/line_parser.rs

@@ -142,7 +142,7 @@ impl LineParser {
         if !has_commands {
             if has_expectations {
                 bail!(
-                    "line {}: testcase output expectation(s) given, but no shell expression specified",
+                    "line {}: testcase output expectation(s) given, but no shell expression specified. Did you forget to prefix the command with '$'?",
                     line_index + 1
                 )
             }