Tweak unknown character message, add test

Author: chenglou

syntax/src/res_diagnostics.ml

@@ -70,7 +70,7 @@ let explain t =
       "  If you're trying to dereference a mutable value, use `myValue.contents` instead.\n" ^
       "  To concatenate strings, use `\"a\" ++ \"b\"` instead."
     | _ ->
-      "I have no idea what this character means…"
+      "Not sure what to do with this character."
     end
   | Expected {context; token = t} ->
     let hint = match context with

syntax/tests/parsing/errors/scanner/__snapshots__/parse.spec.js.snap

@@ -1,5 +1,27 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`badCharacter.js 1`] = `
+"=====Parsetree==========================================
+let 1 = [%rescript.exprhole ]
+=====Errors=============================================
+
+  Syntax error!
+  parsing/errors/scanner/badCharacter.js:1:5  
+  1 │ let $ = 1
+  
+  Not sure what to do with this character.
+
+
+  Syntax error!
+  parsing/errors/scanner/badCharacter.js:1:5-7  
+  1 │ let $ = 1
+  
+  I was expecting a name for this let-binding. Example: \`let message = \\"hello\\"\`
+
+
+========================================================"
+`;
+
 exports[`escapeSequence.js 1`] = `
 "=====Parsetree==========================================
 let x = \\"\\\\\\\\0\\"

syntax/tests/parsing/errors/scanner/badCharacter.js

@@ -0,0 +1 @@
+let $ = 1