Update [getErrorMessage] to avoid throwing away useful information.

Update [getErrorMessage] to avoid throwing away useful information.

Many built-in exception types don't include some relevant information
in their exceptions' [message] property, so this CL goes back to using
[toString] to get string representations of exceptions. The unsightly
exception prefixes are now removed via regular expression.

R=rnystrom@google.com
BUG=

Committed: https://code.google.com/p/dart/source/detail?r=32045

[Bob Nystrom, 2014-01-27 22:59:00]

lgtm

[nweiz, 2014-01-28 00:14:23]

Committed patchset #1 manually as r32045 (presubmit successful).