drm_hwcomposer: Improve error messages.

- Don't negate errno before passing to strerror, at least on
  bionic this results in "Unknown error -x" instead of the actual
  error.
- Fix another error message to print strerror(errno) instead of the
  returned fd which will always be -1.
- Fix another error message to call strerror for consistency.

Signed-off-by: Peter Collingbourne <pcc@google.com>
diff --git a/drm/drmdevice.cpp b/drm/drmdevice.cpp
index bcb9ddd..bef41d8 100644
--- a/drm/drmdevice.cpp
+++ b/drm/drmdevice.cpp
@@ -126,7 +126,7 @@
   /* TODO: Use drmOpenControl here instead */
   fd_.Set(open(path, O_RDWR));
   if (fd() < 0) {
-    ALOGE("Failed to open dri- %s", strerror(-errno));
+    ALOGE("Failed to open dri %s: %s", path, strerror(errno));
     return std::make_tuple(-ENODEV, 0);
   }