patch 9.0.0150: error for using #{ in an expression is a bit confusing

Problem:    Error for using #{ in an expression is a bit confusing.
Solution:   Mention that this error is only given for an expression.
            Avoid giving the error more than once. (closes #10855)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index df65d72..f30cd8d 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -2823,6 +2823,8 @@
   v9.CheckDefAndScriptFailure(["var x = 'a' .. #{a: 1}"], 'E1170:', 1)
   v9.CheckDefAndScriptFailure(["var x = true ? #{a: 1}"], 'E1170:', 1)
 
+  v9.CheckDefAndScriptFailure(["var x = 'a'", " #{a: 1}"], 'E1170:', 1)
+
   v9.CheckDefAndScriptFailure(["var x = {a:8}"], 'E1069:', 1)
   v9.CheckDefAndScriptFailure(["var x = {a : 8}"], 'E1068:', 1)
   v9.CheckDefAndScriptFailure(["var x = {a :8}"], 'E1068:', 1)