Raise on init, parse, render, or write fails

If jinja template initialization, parsing, rendering, or writing fails,
we used to sometimes log an error message, then sometimes continue
(ignoring the failure), sometimes reraise the exception, and sometimes
fail without raising the exception.  With this change, we always raise
the exception so we print the traceback and fail the build right away.

Change-Id: I04a36c4c8c69dbd08e9ea677983dbe00fa3a67a3

tools/www-generator.py

@@ -434,6 +434,7 @@ def render_template(environment, project_data, regular_repos, infra_repos,
             fh.write(output.encode('utf8'))
     except (IOError, OSError, UnicodeEncodeError) as e:
         logger.error("writing %s failed: %s" % (target_file, e))
+        raise
 
 
 def main():
@@ -457,7 +458,7 @@ def main():
         environment = jinja2.Environment(loader=loader)
     except Exception as e:
         logger.error("initialising template environment failed: %s" % e)
-        return 1
+        raise
 
     if args.skip_render:
         return 0