use diagnostic positions in exceptions

[hnampally]

This PR is for #4561, updates the diagnostics function in the exceptions.hpp to use diagnostic positions in exceptions.

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Pull request checklist

Read the Contribution Guidelines for detailed information.

• Changes are described in the pull request, or an existing issue is referenced.
• The test suite compiles and runs without error.
• Code coverage is 100%. Test cases can be added by editing the test suite.
• The source code is amalgamated; that is, after making changes to the sources in the include/nlohmann directory, run make amalgamate to create the single-header files single_include/nlohmann/json.hpp and single_include/nlohmann/json_fwd.hpp. The whole process is described here.

[coveralls]

coverage: 99.186% (+0.001%) from 99.185%
when pulling 5c22738 on hnampally:develop
into bdb8d2b on nlohmann:develop.

[nlohmann]

The coverage job is now fixed in the develop branch. Please update.

[nlohmann]

There is a problem building the documentation:

Error reading page 'api/macros/json_diagnostic_positions.md': Snippet at path 'examples/diagnostic_positions_exception.cpp' could not be found

Pleae check if you can build the documentation locally.

[nlohmann]

Looks good to me.

[nlohmann]

Thanks!