[Supersize] Add strict check for missing tool-prefix.

When Supersize assigns tool-prefix by default, but fails to find
the tools needed, previously it quietly falls back to using other
tools. This CL changes makes this a hard failure instead. For
LLVM, also prints instruction on how to download the required
tools, now that download_objdump.py is ready.

Bug: 723798
Change-Id: Icc7e02deca9800dc7fbf30fbe6314a98c019be25
Reviewed-on: https://chromium-review.googlesource.com/794104
Commit-Queue: Samuel Huang <huangs@chromium.org>
Reviewed-by: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519865}

tools/binary_size/libsupersize/path_util.py

@@ -108,9 +108,11 @@ class ToolPrefixFinder(_PathFinder):
         if os.path.isfile(ret + _SAMPLE_TOOL_SUFFIX):
           return ret
         else:
-          logging.warn('Invalid default tool-prefix: %s', ret)
-          # TODO(huangs): For LLD, print more instruction on how to download
-          # or build the required tools.
+          err_lines = ['tool-prefix not found: %s' % ret]
+          if ret.endswith('llvm-'):
+            err_lines.append('Probably need to run: '
+                             'tools/clang/scripts/download_objdump.py')
+          raise Exception('\n'.join(err_lines))
     from_path = distutils.spawn.find_executable(_SAMPLE_TOOL_SUFFIX)
     if from_path:
       return from_path[:-7]