Script crashes with "TypeError: int() argument must be a string or a number, not 'NoneType'" message

Fix bisect-builds.py crash when no bad build is given and GetSVNRevisionFromGitHash returns None.
Script crashes with "TypeError: int() argument must be a string or a number, not 'NoneType'" message.
The fix is to use 'raise ValueError' instead of returning None.

BUG=None
NOTRY=True

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284953

[Robert Sesek, 2014-07-23 14:26:23]

LGTM

Please make the first line of your CL description a short summary of the change.
E.g. "Fix bisect-builds.py when X, Y, Z." Then the your current CL description
can be the longer description, which is good.

[pshenoy, 2014-07-23 16:38:03]

On 2014/07/23 14:26:23, rsesek wrote:
> LGTM
> 
> Please make the first line of your CL description a short summary of the
change.
> E.g. "Fix bisect-builds.py when X, Y, Z." Then the your current CL description
> can be the longer description, which is good.

Done. Thanks.

[pshenoy, 2014-07-23 16:39:02]

The CQ bit was checked by pshenoy@chromium.org

[commit-bot: I haz the power, 2014-07-23 16:41:29]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/pshenoy@chromium.org/408253004/1

[commit-bot: I haz the power, 2014-07-23 16:44:18]

Change committed as 284953