Fixed wrong behavior in method search_s in BaseLdap class.

Variable 'py_result' can be not initialized if self.page_size > 0
because it's initialized only in the 'else' block.

Closes bug 1305056
Change-Id: Ib9cfb2c03279d97ec0c1a4f8cb45fe5b568b9d7a
Adding more descriptive error message

Keystone Trusts API sends non descriptive error message for requests with
invalid trust id.
The error message only contains invalid_trust_id  instead of printing:
"Could not find trust: invalid_trust_id"
Setting kwarg "trust_id" in the exception.

Closes-Bug: #1305272
Change-Id: I35cab35ab90cf66ce7124dea142013bec9eefda9