Improve conversion to BuildError

[mwichmann]

In some cases, the conversion of another exception to a BuildError could yield undesired results: if the exception was a OSError/IOError, and some fields were initially set to None, then the return code and error string were set to None, rather than the SCons defaults (which are return code 2, and the string set originally in the exception).

A unit test is added which attempts to build an OSError in the way that the case "in the wild" does - confirmed failing to pick up defaults without the change.

Fixes #4530

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt (and read the README.rst)
• [X] I have updated the appropriate documentation