docs: Convert ImageInput-errorchecking examples from docs to tests

Description

Convert error checking C++ and Python examples from the "imageinput" chapter into tests within the "docs-examples" testsuites (https://github.com/AcademySoftwareFoundation/OpenImageIO/issues/3992).

[x] I have read the contribution guidelines.
[x] I have updated the documentation, if applicable. (Check if there is no need to update the documentation, for example if this is a bug fix that doesn't change the API.)
[x] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
[x] If I added or modified a C++ API call, I have also amended the corresponding Python bindings (and if altering ImageBufAlgo functions, also exposed the new functionality as oiiotool options).
[x] My code follows the prevailing code style of this project. If I haven't already run clang-format before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.