Remove bogus std::error_code returns form SectionRef.

There are two methods in SectionRef that can fail:

* getName: The index into the string table can be invalid.
* getContents: The section might point to invalid contents.

Every other method will always succeed and returning and std::error_code just
complicates the code. For example, a section can have an invalid alignment,
but if we are able to get to the section structure at all and create a
SectionRef, we will always be able to read that invalid alignment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219314 91177308-0d34-0410-b5e6-96231b3b80d8