[Qt5][C++] Fixed QByteArray::toUtf8 with FILE as request body. Included <stdexcept>.

[igor-sirotin]

• Added '#include ' where std::runtime_error is used
  I get a build error with MSVC 16.6.30114.105 without this include:

error C2039: 'runtime_error': is not a member of 'std'
error C3861: 'runtime_error': identifier not found

• Fixed QByteArray::toUtf8 with FILE as request body
  In api-body.mustache there's .toUtf8() call on QString output to get a valid QByteArray for the following input.request_body.append(const QByteArray&). This one is not needed when output is obtained already as QByteArray.

• Updated related samples

• Updated generator docs

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ping @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[igor-sirotin]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

@wing328, oops, my fault!
Added this e-mail to my GitHub account. Thanks for the tip.

[etherealjoy]

@igor-sirotin
Changes looks good to me. Were you able to compile this also on Linux?

[igor-sirotin]

@etherealjoy, no, I haven't done that.
Totally relied on CI, because the changes aren't OS-dependent.