Issuer schemes can get out-of-sync after interrupted scheme update

[ivard]

Stack trace:

String: Cannot initialize client: Hash of /var/mobile/Containers/Data/Application/***/Library/v2/irma_configuration/irma-demo/description.xml does not match scheme manager index
  File "*errors.errorString", in Cannot initialize client: Hash of /var/mobile/Containers/Data/Application/***/Library/v2/irma_configuration/irma-demo/description.xml does not match scheme manager index
  File "schemes.go:687", in (0x105385dd4)
  File "schemes.go:675", in (0x105385c8c)
  File "schemes.go:342", in (0x10538371c)
  File "irmaconfig.go:156", in (0x105369f90)
  File "common.go:245", in (0x1051d04f8)
  File "common.go:225", in (0x1053699ac)
  File "irmaconfig.go:155", in (0x105369959)
  File "irmaconfig.go:227", in (0x10536a3dc)
  File "client.go:176", in (0x10539d18c)
  File "bridge.go:150", in (0x1053ba3e0)
  File "go_irmagobridgemain.go:196", in (0x1053c29ac)
  File "unparsed", in _cgo_gotypes.go:296 (0x1053c3734)
  File "cgocall.go:316", in (0x104f6a4e4)
  File "cgocall.go:235", in (0x104f6a218)
  File "asm_arm64.s:1087", in (0x104fcb980)
  File "asm_arm64.s:1165", in (0x104fcba54)