The doxygen plugin is executed even if a build failed, see log below. In my
opinion this should not happen (or at least, it should be an option on the
config page whether to spawn the doxygen plugin after a failed build). In this
particular case, Hudson is launching a nightly build by calling an ant script
that compiles the code and runs the doxygen documentation generation. However,
if the build fails, the documentation is not generated and thus, the plugin will
fail to copy the docs (as you can see in the following log):

[...]

BUILD FAILED
C:_CI\jobs\Test\workspace\build.xml:140: exec returned: 1

Total time: 16 minutes 22 seconds
[locks-and-latches] Releasing all the locks
[locks-and-latches] All the locks released
[WARNINGS] Parsing warnings in log file...
[WARNINGS] Found 185 annotations (31 new, 0 high, 185 normal, 0 low)
[WARNINGS] Not changing build status, since no threshold has been exceeded
Publishing Doxygen HTML results.
The determined Doxygen directory is 'C:_CI\jobs\Test\workspace_doc/html'.
Failure to copy the generated doxygen html documentation at '_doc/html' to
'C:_CI\jobs\Test\doxygen\html'
Email was triggered for: Failure
There are 1 triggered emails.
[...]
Sending email for trigger: Failure
Finished: FAILURE