run-webkit-tests: fix crash when trying to read a log that did not exist.

run-webkit-tests: fix crash when trying to read a log that did not exist.

During error recovery after failing to start a server, there might actually
not be an error log where we think there should be one.

TBR=eseidel@chromium.org
BUG=378444
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=179077

[Dirk Pranke, 2014-07-28 22:10:26]

The CQ bit was checked by dpranke@chromium.org

[commit-bot: I haz the power, 2014-07-28 22:11:39]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dpranke@chromium.org/421413002/1

[commit-bot: I haz the power, 2014-07-28 23:59:04]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_blink_compile_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_compile_dbg/bu...)
  mac_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_rel/builds/17410)

[commit-bot: I haz the power, 2014-07-29 00:39:25]

Change committed as 179077