Extract hardcoded error strings into a single place and replace them with enum.

src/compiler.h

Index: src/compiler.h
diff --git a/src/compiler.h b/src/compiler.h
index d36e488b174f9fbed7204ced0d5d6836c5afce1b..50053e562512d29ce3177816af11eaf101206e51 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -258,8 +258,8 @@ class CompilationInfo {
     SaveHandle(&script_);
   }
 
-  const char* bailout_reason() const { return bailout_reason_; }
-  void set_bailout_reason(const char* reason) { bailout_reason_ = reason; }
+  BailoutReason bailout_reason() const { return bailout_reason_; }
+  void set_bailout_reason(BailoutReason reason) { bailout_reason_ = reason; }
 
   int prologue_offset() const {
     ASSERT_NE(kPrologueOffsetNotSet, prologue_offset_);
@@ -412,7 +412,7 @@ class CompilationInfo {
     }
   }
 
-  const char* bailout_reason_;
+  BailoutReason bailout_reason_;
 
   int prologue_offset_;