safe_mutex issues must be assertions in debug binary

[Philip Stoev]

All safe_mutex warnings must cause assertions in a debug binary. If they are simply reported in the error log with no other indication, they are very difficult to detect in an automatic test.

[Michael Widenius (monty)]

Personally I don't want to have an assert here, as I prefer to get all mutex error warnings collected instead of getting a crash.
I have however added a new option --debug-assert-on-error, which should be used by automatic tests that wants asserts for things like this.
Setting this new variable also sets --debug-assert-if-crashed-table.