Return an Err on shape mismatch instead of panic

[krlohnes]

This way we let the user of the library decide how to handle the issue.

This is a fix for #76

[codecov-commenter]

Codecov Report

Merging #77 (788bd3a) into master (1948683) will increase coverage by 0.26%.
The diff coverage is 52.43%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   14.58%   14.85%   +0.26%     
==========================================
  Files          18       19       +1     
  Lines         960     1212     +252     
==========================================
+ Hits          140      180      +40     
- Misses        820     1032     +212     

Impacted Files	Coverage Δ
onnxruntime/src/lib.rs	42.55% <0.00%> (+5.20%)	⬆️
onnxruntime/src/session.rs	0.00% <0.00%> (ø)
onnxruntime/src/tensor/ort_tensor.rs	79.16% <75.47%> (-3.19%)	⬇️
onnxruntime/src/error.rs	62.50% <100.00%> (+5.35%)	⬆️
onnxruntime/src/download.rs	0.00% <0.00%> (ø)
...runtime-sys/src/generated/macos/x86_64/bindings.rs	0.00% <0.00%> (ø)
...untime/src/download/vision/image_classification.rs	0.00% <0.00%> (ø)
...ime/src/download/language/machine_comprehension.rs	0.00% <0.00%> (ø)
... and 4 more

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[nbigaouette]

Thank you!