[llvm-mc] More meaningful error if input file doesn't exist.

[llvm-mc] More meaningful error if input file doesn't exist.

Before we just printed on stderr the program name. Now at least we
print the name of the file that doesn't exist. There's probably room
for improvement of error handling in llvm-mc (and the tools in general),
but still this is a step forward.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@247518 91177308-0d34-0410-b5e6-96231b3b80d8