Fix generate_emma_html.py failing due to 0-length .em files

This also makes it return non-zero when the .html is not actually
created.

BUG=587983

Review URL: https://codereview.chromium.org/1711983002

Cr-Original-Commit-Position: refs/heads/master@{#376348}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: ae67350c3a10889adb6bf3f47002227546245737

android/generate_emma_html.py

@@ -58,6 +58,9 @@ def main():
 
   coverage_files = _GetFilesWithExt(options.coverage_dir, 'ec')
   metadata_files = _GetFilesWithExt(options.metadata_dir, 'em')
+  # Filter out zero-length files. These are created by emma_instr.py when a
+  # target has no classes matching the coverage filter.
+  metadata_files = [f for f in metadata_files if os.path.getsize(f)]
   print 'Found coverage files: %s' % str(coverage_files)
   print 'Found metadata files: %s' % str(metadata_files)
 
@@ -87,6 +90,10 @@ def main():
     for f in coverage_files:
       os.remove(f)
 
+  # Command tends to exit with status 0 when it actually failed.
+  if not exit_code and not os.path.exists(options.output):
+    exit_code = 1
+
   return exit_code