Integrate Exceptions Better with PhpStorm

[oleibman]

Fix #3754. Because PhpSpreadsheetException extends Exception, PhpStorm warns that calls to setValueExplicit and setValue, among others, do not handle the exception, because PhpStorm treats Exception, by default, as "checked". On the other hand, PhpStorm treats RunTimeException as "unchecked" so won't flag such calls. It is reasonable to let PhpSpreadsheetException extend RuntTimeException, and will eliminate the problem, without having to wrap code in all-but-useless try-catch blocks (e.g. for setValue, the code would raise an exception only if you try to set the cell's value to an unstringable object).

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests
• IDE integration

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change and a link to the pull request if applicable
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.