bootstrap: handle snapshot errors gracefully

This patch refactors the SnapshotBuilder::Generate() routines
so that when running into errors during the snapshot building
process, they can exit gracefully by printing the error
and return a non-zero exit code. If the error is likely to
be caused by internal scripts, the return code would be 12,
if the error is caused by user scripts the return code would
be 1. In addition this refactors the generation of embedded
snapshots and directly writes to the output file stream
instead of producing an intermediate string with string
streams.

src/node_snapshot_builder.h

@@ -15,13 +15,14 @@ struct SnapshotData;
 
 class NODE_EXTERN_PRIVATE SnapshotBuilder {
  public:
-  static std::string Generate(const std::vector<std::string> args,
-                              const std::vector<std::string> exec_args);
+  static int Generate(std::ostream& out,
+                      const std::vector<std::string> args,
+                      const std::vector<std::string> exec_args);
 
   // Generate the snapshot into out.
-  static void Generate(SnapshotData* out,
-                       const std::vector<std::string> args,
-                       const std::vector<std::string> exec_args);
+  static int Generate(SnapshotData* out,
+                      const std::vector<std::string> args,
+                      const std::vector<std::string> exec_args);
 
   // If nullptr is returned, the binary is not built with embedded
   // snapshot.