Detect compile errors when setting breakpoint

Detect compile errors when setting breakpoint

When the VM is started with the —-debug option, it compiles the main
function eagerly to set a one-shot breakpoint. So far we silently
dropped errors during the compilation, which is wrong. This change
propagates the error to the caller.

Fixes issue https://code.google.com/p/dart/issues/detail?id=20558

R=regis@google.com

Committed: https://code.google.com/p/dart/source/detail?r=39390

[regis, 2014-08-19 20:26:12]

LGTM

[hausner, 2014-08-19 22:34:11]

Committed patchset #1 manually as r39390 (presubmit successful).