[llvm-pdbdump] Better error handling.

[llvm-pdbdump] Better error handling.

Previously it was impossible to distinguish between "There is
no PDB implementation for this platform" and "I tried to load
the PDB, but couldn't find the file", making it hard to figure
out if you built llvm-pdbdump incorrectly or if you just mistyped
a file name.

This patch adds proper error handling so that we can know exactly
what went wrong.

llvm-svn: 230868