I was having a problem passing a unicode string to LdtpExecutionError:

import ldtp

try:
    raise ldtp.LdtpExecutionError(u"testraise\u221a")
except Exception, e:
    print unicode(e)

The exceptions in LDTP seem to be copied from examples in the Python documentation for overriding exceptions. Resultantly, they store the message in a variable called value rather than in the args variable like the base class. This wouldn't be a problem except that the base class has its own __unicode__ method which depends on the args variable, so calling unicode() on an LDTP exception returns a null string. 

Changing exceptions to just use the Exception class as-is resolves this issue. The attached patch also makes LDTP exceptions inherit from ldtp.Error so all LDTP exceptions can be caught by catching ldtp.Error. I think this was the original design philosophy but it wasn't implemented.

There were quite a few places where an exception handler used LdtpExecutionError.value - these are updated in the 0002 patch.