Switch FileWriter::Write to boolean result code.

FileWriter::Write returned either the number of bytes written, or a negative
error code. No clients were doing anything with the result other than checking
for success or failure, and many clients were considering any non-zero result
success.

So, I changed the API to return less information, but just the information the
clients needed. Success or failure.

BUG=chromium-os:8521
TEST=Unittests

Change-Id: I51513d6aa7b704dc27fb90d5fae4dc7118a3f86c
Reviewed-on: https://gerrit.chromium.org/gerrit/11532
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Don Garrett <dgarrett@chromium.org>
Commit-Ready: Don Garrett <dgarrett@chromium.org>
diff --git a/delta_performer.h b/delta_performer.h
index 5c1f3ab..dd8f8d8 100644
--- a/delta_performer.h
+++ b/delta_performer.h
@@ -52,9 +52,9 @@
   // Open()ed again.
   int Open(const char* path, int flags, mode_t mode);
 
-  // Wrapper around write. Returns bytes written on success or
-  // -errno on error.
-  ssize_t Write(const void* bytes, size_t count);
+  // Wrapper around write. Returns true if all requested bytes
+  // were written, or false on any error, reguardless of progress.
+  bool Write(const void* bytes, size_t count);
 
   // Wrapper around close. Returns 0 on success or -errno on error.
   // Closes both 'path' given to Open() and the kernel path.