Check for path errors eagerly during load/save (#263)

[IanButterworth]

This adds more eager checking for path errors during load & save for clearer error messages.
Fixes #263

---

Loading an a nonexistant file

Before:

julia> FileIO.load("fake/fake.png")
Errors encountered while loading "fake/fake.png".
All errors:
===========================================
Failed to open fake/fake.png
===========================================
ArgumentError: Package QuartzImageIO not found in current path:
- Run `import Pkg; Pkg.add("QuartzImageIO")` to install the QuartzImageIO package.

===========================================
ArgumentError: Package ImageMagick not found in current path:
- Run `import Pkg; Pkg.add("ImageMagick")` to install the ImageMagick package.

===========================================

Fatal error:
ERROR: Failed to open fake/fake.png
...

This PR:

julia> FileIO.load("fake/fake.png")
ERROR: ArgumentError: No file exists at given path: fake/fake.png
Stacktrace:
...

---

Trying to save into a directory that doesn't exist

Before:

julia> FileIO.save("fake_dir/image.png", rand(UInt8,10,10))
Errors encountered while saving "fake_dir/image.png".
All errors:
===========================================
Could not open fake_dir/image.png for writing
===========================================
ArgumentError: Package QuartzImageIO not found in current path:
- Run `import Pkg; Pkg.add("QuartzImageIO")` to install the QuartzImageIO package.

===========================================
ArgumentError: Package ImageMagick not found in current path:
- Run `import Pkg; Pkg.add("ImageMagick")` to install the ImageMagick package.

===========================================
ArgumentError: Argument does not support conversion to png.
===========================================

Fatal error:
ERROR: Could not open fake_dir/image.png for writing
Stacktrace:
...

This PR:

julia> FileIO.save("fake_dir/image.png", rand(UInt8,10,10))
ERROR: ArgumentError: Directory of file path does not exist: fake_dir/image.png
Stacktrace:
...

UPDATE: Now mkpath() is run on the parent directory, if it doesn't exist and no error is thrown

---

Trying to save to a path that's a directory

Before

julia> FileIO.save(format"PNG", "existing_dir", rand(UInt8,10,10))
Errors encountered while saving "existing_dir".
All errors:
===========================================
Could not open existing_dir for writing
===========================================
ArgumentError: Package QuartzImageIO not found in current path:
- Run `import Pkg; Pkg.add("QuartzImageIO")` to install the QuartzImageIO package.

===========================================
ArgumentError: Package ImageMagick not found in current path:
- Run `import Pkg; Pkg.add("ImageMagick")` to install the ImageMagick package.

===========================================
ArgumentError: Argument does not support conversion to png.
===========================================

Fatal error:
ERROR: Could not open existing_dir for writing
Stacktrace:
...

This PR:

julia> FileIO.save(format"PNG", "existing_dir", rand(UInt8,10,10))
ERROR: ArgumentError: Given file path is a directory: existing_dir
Stacktrace:
...

cc. @johnnychen94 @timholy

[IanButterworth]

By the way, is ArgumentError the best option to throw here?

[codecov]

Codecov Report

Merging #265 into master will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #265      +/-   ##
==========================================
+ Coverage   76.02%   76.31%   +0.28%     
==========================================
  Files           8        8              
  Lines         413      418       +5     
==========================================
+ Hits          314      319       +5     
  Misses         99       99              

Impacted Files	Coverage Δ
src/loadsave.jl	86.84% <100.00%> (+0.92%)	⬆️

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[johnnychen94]

@ianshmean Thanks! I've recently seen a lot of people bit by this obscure error message, even for core devs in JuliaImages. For example, JuliaImages/Images.jl#888 JuliaImages/ImageEdgeDetection.jl#7

One reason I didn't put my hands on this is that I feel it over-eager to check whether the directory exists when saving images in FileIO; I feel that should depend on backends implementations. The other two checks look good to me.

[johnnychen94]

My best guess is that some backends might support path creation before writing, so checking if the directory exists is perhaps too eagerly for a general IO frontend.

Checking whether the file path is a directory looks good to me.

[IanButterworth]

Perhaps a solution could be to always mkpath() before passing to the backends? It's been mentioned before that that would be nice default behavior.

[johnnychen94]

Not sure how others think, mkpath by default sounds a good solution to me

[Octogonapus]

Perhaps a solution could be to always mkpath() before passing to the backends? It's been mentioned before that that would be nice default behavior.

I would also like this to be the default.

[IanButterworth]

Ok, great. I've made that the case now

[johnnychen94]

LGTM

[IanButterworth]

Should we wait for more reviews?

[timholy]

I think it's possible we'll be troubled by the path-creation, but this is undeniably better and might be perfect. Let's try it and see how we like it.

I think this merits a minor version bump, though? As a new feature? (That won't be breaking as we're over 1.0.)

[IanButterworth]

Minor bump sounds right. Thanks!

[timholy]

Thanks for fixing this, BTW!