Work with standard GOPATH

I was getting incorrect behavior from `goveralls` because it was expecting this kind of file layout:

```
$GOPATH/
  src/
    foobar/
      foobar.go
```

Whereas I use this kind of file layout, which is increasingly "standard" in the community:

```
$GOPATH/
  src/
    github.com/
      jmcvetta/
        foobar/
          foobar.go
```

This problem was fixed by changing a regular expression.

Also make `goveralls` exit with an error if it cannot open a source file, such as could happen if user runs it from somewhere other than the package directory.