Add internal documentaion about exception-safe functions

doc/internal/exception_safe.md

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+# Exception-Safe functions
+
+The public API dataservices functions emit exceptions in general when an error occurs
+or a limiting condition is met (e.g. quotas are exceeded).
+
+For each public function `x` we have a internal function named `_x_exception_safe` which
+acts as a wrapper to the public function, with the same signature, but captures
+exceptions generated during its execution (except those due to incomplete configuration or
+authentication issues) and returns NULL or empty set values in those cases.
+
+## Intended Use
+
+These functions are useful in cases when it is undesirable to rollback a transaction.
+Fo example if a table is geocoded with:
+
+```sql
+UPDATE table SET the_geom=cdb_geocode_street_point(user,NULL,address,city,NULL,country);
+```
+
+In case of the user geocoding quota being exhausted mid-process, the user could
+incur in external service expenses but any geocoded data would be lost due to the
+transaction rollback.
+
+We can avoid the problem using the corresponding exception-safe function:
+
+```sql
+UPDATE table SET the_geom=_cdb_geocode_street_point_exception_safe(user,NULL,address,city,NULL,country);
+```
+
+# Addition Information
+
+See https://github.com/CartoDB/dataservices-api/issues/314 for more information.