More informative output when tests fail in summarize_system_checks

**Is your feature request related to a problem? Please describe.**

When running:

``` r
ropsec::summarize_system_checks()
#> Registered S3 method overwritten by 'openssl':
#> method from
#> print.bytes Rcpp
#> ✔ | OK F W S | Context
#>
⠏ | 0 | SSH Configuration - existence
✔ | 1 | SSH Configuration - existence
#>
⠏ | 0 | SSH Configuration - keys
✔ | 2 | SSH Configuration - keys
#>
⠏ | 0 | SSH Configuration - key size
✔ | 1 | SSH Configuration - key size
#>
⠏ | 0 | GPG Existence
✔ | 1 | GPG Existence
#>
⠏ | 0 | macOS requires password after sleep or screen saver kicks in
✖ | 0 1 1 | macOS requires password after sleep or screen saver kicks in
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#> macos-simple-test.R:32: warning: macOS requires password after sleep or screen saver kicks in
#> running command 'defaults read com.apple.screensaver askForPassword' had status 1
#>
#> macos-simple-test.R:33: failure: macOS requires password after sleep or screen saver kicks in
#> x == 1 isn't true.
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#>
⠏ | 0 | Firewall is enabled
✔ | 1 | Firewall is enabled
#>
⠏ | 0 | Gatekeeper is enabled
✔ | 1 | Gatekeeper is enabled
#>
⠏ | 0 | ~/.Rprofile permissions are sane (if ~/.Rprofile exists)
✖ | 0 1 | ~/.Rprofile permissions are sane (if ~/.Rprofile exists)
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#> macos-simple-test.R:52: failure: ~/.Rprofile permissions are sane
#> ==... isn't true.
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#>
⠏ | 0 | ~/.Renviron permissions are sane (if ~/.Renviron exists)
✖ | 0 1 | ~/.Renviron permissions are sane (if ~/.Renviron exists)
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#> macos-simple-test.R:59: failure: ~/.Renviron permissions are sane
#> ==... isn't true.
#> ─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
#>
#> ══ Results ══════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════════
#> Duration: 0.3 s
#>
#> OK: 7
#> Failed: 3
#> Warnings: 1
#> Skipped: 0
```

^{Created on 2019-05-08 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1)}

both `.Renviron` and `.Rprofile` checks fail. However, the output of the test and documentation doesn't have much information on what the test failure means and what to do about it, at least for someone like me who is not very knowledgable on this stuff. By looking at the source code for the check I figured out it's something to do with file write permissions? But still not sure what the best approach to solve it is and why.

**Describe the solution you'd like**
A more informative message during test failure and potentially more detail in the function documentation on what to do to resolve identified issues. Importantly, it's a great opportunity to educate folks like me about the why of all this too.

### Session info

``` r
sessionInfo()
#> R version 3.6.0 (2019-04-26)
#> Platform: x86_64-apple-darwin15.6.0 (64-bit)
#> Running under: macOS Mojave 10.14.3
#>
#> Matrix products: default
#> BLAS: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRblas.0.dylib
#> LAPACK: /Library/Frameworks/R.framework/Versions/3.6/Resources/lib/libRlapack.dylib
#>
#> locale:
#> [1] en_GB.UTF-8/en_GB.UTF-8/en_GB.UTF-8/C/en_GB.UTF-8/en_GB.UTF-8
#>
#> attached base packages:
#> [1] stats graphics grDevices utils datasets methods base
#>
#> loaded via a namespace (and not attached):
#> [1] compiler_3.6.0 magrittr_1.5 tools_3.6.0 htmltools_0.3.6
#> [5] yaml_2.2.0 Rcpp_1.0.1 stringi_1.4.3 rmarkdown_1.12
#> [9] highr_0.8 knitr_1.22 stringr_1.4.0 xfun_0.6
#> [13] digest_0.6.18 evaluate_0.13
```

^{Created on 2019-05-08 by the [reprex package](https://reprex.tidyverse.org) (v0.2.1)}