When you specify a filename to `process save core`, we'll write it in
the current working directory. In Xcode the CWD is `/` and you can't
generally write there.
```
(lldb) process save-core --style full foo
error: failed to save core file for process: Read-only file system
```
This PR improves the error message by including the output file when we
can't save core. However, just printing the filename isn't that much
more helpful, because FileSystem::Resolve only makes a path absolute if
it exists.
```
error: failed to save core file for process to 'foo': Read-only file system
```
Therefore I also modified the interface to add a flag to force resolve a
potentially non-exist path.
```
error: failed to save core file for process to '/foo': Read-only file system
```
859052a34e