Parse <error> and <failure> tags as well

[florianpilz]

Currently, just the tags <system-out> and <system-err> are parsed to look for attachments. I propose search in <error> and <failure> tags as well, because when using a different test runner, it may use the tags differently.

To give a concise example: We are using the zope.testrunner to test our Python projects. The generated XML uses the same tags as allowed by JUnit Test Cases, however, errors and tracebacks are written to <error> and <failure>, thus the problem.

[jglick]

Please use JIRA.