#663 - show error messages on error

...adlocker8/budgetmaster/migration/MigrationController.java

.../deadlocker8/budgetmaster/migration/MigrationService.java

.../de/deadlocker8/budgetmaster/migration/MigrationTask.java

...terServer/src/main/resources/languages/base_de.properties

...terServer/src/main/resources/languages/base_en.properties

BudgetMasterServer/src/main/resources/static/js/migration.js

...rServer/src/main/resources/templates/migration/status.ftl

...src/main/resources/templates/migration/statusFragment.ftl