Make exceptions loading extension schemas non-fatal. (#144)
Make exceptions loading extension schemas non-fatal. (#144)

* Make exceptions loading extension schemas non-fatal.

Also cache results of check_exists().

common-workflow-language/schema_salad,common-workflow-language/schema_salad,common-workflow-language/schema_salad,common-workflow-language/schema_salad,common-workflow-language/schema_salad,common-workflow-language/schema_salad,common-workflow-language/schema_salad