Improve server error messages

[LPGhatguy]

Some of the error messages Rojo has are pretty bad; any place where Rojo can panic, those error messages should be customized in each case to make things more obvious.

For example, if a partition target is missing on the filesystem, you get:

$ rojo serve
Using project from ...
Server listening on port 8000
thread '<unnamed>' panicked at 'Unable to watch path!: Generic("Input watch path is neither a file nor a directory.")', src\libcore\result.rs:906:4
note: Run with `RUST_BACKTRACE=1` for a backtrace.