Chore: improve arrow-body-style error message (refs #5498) (#9718)

#5498 gets more than twice as
much traffic as any of our other issues. My interpretation of this fact
is that people are often confused by errors from the `arrow-body-style`
rule, and end up searching and finding that issue. This commit updates
the error message for `arrow-body-style` to be more specific about how
to fix the problem.

lib/rules/arrow-body-style.js

@@ -109,10 +109,22 @@ module.exports = {
                 }
 
                 if (never || asNeeded && blockBody[0].type === "ReturnStatement") {
+                    let message;
+
+                    if (blockBody.length === 0) {
+                        message = "Unexpected block statement surrounding arrow body; put a value of `undefined` immediately after the `=>`.";
+                    } else if (blockBody.length > 1) {
+                        message = "Unexpected block statement surrounding arrow body.";
+                    } else if (astUtils.isOpeningBraceToken(sourceCode.getFirstToken(blockBody[0], { skip: 1 }))) {
+                        message = "Unexpected block statement surrounding arrow body; parenthesize the returned value and move it immediately after the `=>`.";
+                    } else {
+                        message = "Unexpected block statement surrounding arrow body; move the returned value immediately after the `=>`.";
+                    }
+
                     context.report({
                         node,
                         loc: arrowBody.loc.start,
-                        message: "Unexpected block statement surrounding arrow body.",
+                        message,
                         fix(fixer) {
                             const fixes = [];