[llvm-dis] Improve missing file error message

[keith]

Summary

Previously the error message didn't include the failing path, which made
it hard to tell what went wrong.

llvm/test/tools/llvm-dis/errors.test

+# RUN: not llvm-dis missing-file-path 2>&1 | FileCheck %s --check-prefix=MISSING
+
+# MISSING: error: missing-file-path: No such file or directory

llvm/tools/llvm-dis/llvm-dis.cpp

if (InputFilenames.size() < 1) {
     InputFilenames.push_back("-");
   } else if (InputFilenames.size() > 1 && !OutputFilename.empty()) {
     errs()
         << "error: output file name cannot be set for multiple input files\n";
     return 1;
   }
 
   for (std::string InputFilename : InputFilenames) {
-    std::unique_ptr<MemoryBuffer> MB = ExitOnErr(
-        errorOrToExpected(MemoryBuffer::getFileOrSTDIN(InputFilename)));
+    ErrorOr<std::unique_ptr<MemoryBuffer>> BufferOrErr =
+        MemoryBuffer::getFileOrSTDIN(InputFilename);
+    if (std::error_code EC = BufferOrErr.getError()) {
+      WithColor::error() << InputFilename << ": " << EC.message() << '\n';
+      return 1;
+    }
+    std::unique_ptr<MemoryBuffer> MB = std::move(BufferOrErr.get());
 
     BitcodeFileContents IF = ExitOnErr(llvm::getBitcodeFileContents(*MB));
 
     const size_t N = IF.Mods.size();
 
     if (OutputFilename == "-" && N > 1)
       errs() << "only single module bitcode files can be written to stdout\n";