Enabling overridable error behavior

[mjhenkes]

As discussed in issue #46 I've put together a PR to allow consumers to override the default error behavior of memory-fs.

My use case for this change is that if a file is not found in memory-fs, the error behavior can fall back to search for the file in the standard file system.

[mjhenkes]

regarding the build failure, the npm run cover -- --report lcovonly && npm run lint command runs locally. I'm unsure why it fails in travis :(

[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 0.82%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   96.24%   95.42%   -0.83%     
==========================================
  Files           4        4              
  Lines         293      306      +13     
  Branches       67       73       +6     
==========================================
+ Hits          282      292      +10     
- Misses         11       14       +3

Impacted Files	Coverage Δ
lib/MemoryFileSystem.js	97.78% <91.17%> (-1.28%)	⬇️

---

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[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 0.82%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   96.24%   95.42%   -0.83%     
==========================================
  Files           4        4              
  Lines         293      306      +13     
  Branches       67       73       +6     
==========================================
+ Hits          282      292      +10     
- Misses         11       14       +3

Impacted Files	Coverage Δ
lib/MemoryFileSystem.js	97.78% <91.17%> (-1.28%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 67cec91...06ee2d2. Read the comment docs.

[mjhenkes]

I'm going to fix a couple of things I think are wrong and improve code coverage. I shall be back!