Calling MappingReader.read with an unsupported format results in an undescriptive exception #25
Description
When one of the read methods is passed an unsupported format (apart from MCP), the code throws an IllegalStateException with no message:
mapping-io/src/main/java/net/fabricmc/mappingio/MappingReader.java
Lines 190 to 191 in 597f072
IllegalStateException isn't really appropriate here ("Signals that a method has been invoked at an illegal or inappropriate time."), and the user gets no useful information from the exception since it has no message either.