Replace ReaderError with DynamicError.

ReaderErrorCategory was used only at one place. We now have a
DynamicErrorCategory for this kind of one-time error, so use it.
The calling function doesn't really care the type of an error, so
ReaderErrorCategory was actually dead code.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@224245 91177308-0d34-0410-b5e6-96231b3b80d8