Fix a typo
Merge pull request #2836 from thilinarmtb/fix_docstring_typo

Fix a typo
Added inverses to error functions.

As discussed at
https://groups.google.com/forum/#!topic/sympy/jmV52ve9Cqg
Added tests.
Removed possibly incorrect inverses
Merge pull request #2838 from raoulb/error_inverses

Error inverses
Merge branch 'master' into order-point
Fix Wester test_X8