Generalize Messages to include an error level.
Add a parameter to AddMessageHandler to select which error levels to receive, using a mask (default being just errors, i.e. the current behavior).

BUG=v8:4203
R=dgozman@chromium.org,machenbach@chromium.org,danno@chromium.org,bmeurer@chromium.org,jochen@chromium.org

Review-Url: https://codereview.chromium.org/2526703002
Cr-Commit-Position: refs/heads/master@{#41648}