Browser report fails when `node_modules` are read-only

### Description of a problem

In environments where `node_modules` are read-only browser report isn't created.

Examples of such environments: [nix](https://nixos.org/nix/) based builds, where `node_modules` land in `/nix/store` as read-only (`555` or `444` rights).
Another one might be some stateless build systems, where `node_modules` is passed between build steps as an artifact and also might be read-only.

### How to reproduce:

```
mkdir test && cd test
npm init -y && npm install backstopjs
./node_modules/.bin/backstop init
chmod -w ./node_modules/.bin/backstop/compare/output # simulate read-only case
./node_modules/.bin/backstop test
...

report | 2 Failed
report | Writing browser report
report | Failed writing report with error: undefined # <-- this
```

### Potential cause

I dug a little into the internals and it looks that it's caused by https://github.com/garris/BackstopJS/blob/ddfedcbd2d63fd6108922bf0bfabc31d672fd5c1/core/command/report.js#L26

where `fs.copy` copies `output` with directory rights. Assuming I'm not wrong about how *nix systems handle permissions, in presented case, there's no `write` for `output` directory - thus no new directories `html_report` directory on the project side can be created as part of `copy`.

### Workarounds

One workaround is to create directory structure **before** running tests, something like:

```
rm -rf backstop_data/html_report && mkdir -p backstop_data/html_report/fonts/assets
```

### Summary

Not sure whether it should be considered as a bug, I just wanted to point out that current implementation might be causing some issues.