Fix strict mode function error message

ES2015 allows strict mode block scoped function declarations; weaken
the error message about misuse to allow this.

BUG=v8:2198
LOG=Y
R=adamk

Review URL: https://codereview.chromium.org/1741903002

Cr-Commit-Position: refs/heads/master@{#34339}

src/messages.h

@@ -442,7 +442,7 @@ class CallSite {
   T(StrictEvalArguments, "Unexpected eval or arguments in strict mode")        \
   T(StrictFunction,                                                            \
     "In strict mode code, functions can only be declared at top level or "     \
-    "immediately within another function.")                                    \
+    "inside a block.")                                                         \
   T(StrictOctalLiteral, "Octal literals are not allowed in strict mode.")      \
   T(StrictWith, "Strict mode code may not include a with statement")           \
   T(StrongArguments,                                                           \