Improve error handling in the linker by:

1. Eliminate redundant error messages. LinkInFile and LinkInArchive
   already call the error() method in each case so there's no use
   telling the user again that an item couldn't be linked in.

2. Improve the formatting of error messages (separating content).

3. Change the wording for the warning about unrecognized files. Make
   it clear that the file is being ignored.

llvm-svn: 41121