MDL-30147 do not rely on dml exception type outside of dml layer

The trouble is that dml driver methods (insert, update, select) are not guaranteed to return the same exception class for various db problems and coding style issues. The recommended practice is to catch dml_exception only.

lib/setuplib.php

@@ -657,7 +657,7 @@ function initialise_cfg() {
                 $CFG->{$name} = $value;
             }
         }
-    } catch (dml_read_exception $e) {
+    } catch (dml_exception $e) {
         // most probably empty db, going to install soon
     }
 }