Rename breakOnUnsupportedCmds to throwOnError.

Also, the MathBb-chrome test changed, to what I believe is the correct
result? Not sure why it looked wrong before.

Test plan:
 - `make test`
 - take screenshots, see nothing changed.

test/screenshotter/ss_data.yaml

@@ -106,7 +106,7 @@ SupSubOffsets: \displaystyle \int_{2+3}x f^{2+3}+3\lim_{2+3+4+5}f
 Text: \frac{a}{b}\text{c~ {ab} \ e}+fg
 UnsupportedCmds:
     tex: \err\,\frac\fracerr3\,2^\superr_\suberr\,\sqrt\sqrterr
-    doNotBreak: 1
+    noThrow: 1
     errorColor: "#dd4c4c"
 VerticalSpacing:
     pre: potato<br>blah