Improve proj4j tests to reflect the recent changes and the moved out EPSG resources

[pomadchin]

After separating the project into two, we may get some confusing errors; i.e.

java.lang.IllegalStateException: Unable to access CRS file: proj4/nad/epsg
  at org.locationtech.proj4j.io.Proj4FileReader.readParametersFromFile(Proj4FileReader.java:44)
  at org.locationtech.proj4j.io.Proj4FileReader.getParameters(Proj4FileReader.java:119)
  at org.locationtech.proj4j.CRSFactory.createFromName(CRSFactory.java:83)
  at org.apache.calcite.runtime.ProjectionTransformer.<init>(ProjectionTransformer.java:57)
  at org.apache.calcite.runtime.SpatialTypeFunctions.ST_Transform(SpatialTypeFunctions.java:[1196]

This task is to improve error messages and documentation.

cc @bchapuis

[RoggenRentier]

For anyone coming across this issue when looking up this error message:
The project has been seperated into two and the epsg CRS files are now longer included in the Proj4j library, but in a seperate proj4j-epsg library.
The error is fixed by adding this library to your project.

[pomadchin]

Also PRs are welcome! If there is a way to make this more clear we need it!

[bchapuis]

Yes, we should definitively make things clearer. I see that the README has been updated. What about the following ideas:

Add a clearer error message, such as:

java.lang.IllegalStateException: The proj4j-epsg JAR file is not present in the classpath. This dependency is required to access the proj4/nad/epsg file. Please ensure that the proj4j-epsg JAR is added to your project dependencies.

Hard coding a couple of projection definitions in the core library (e.g. epsg:4326, epsg:3857, etc.). Apparently, the following was acceptable from an Apache License perspective.

https://github.com/apache/incubator-baremaps/blob/8dce39f7d87bdc932c832df5418cc2fdcd1add7f/baremaps-openstreetmap/src/main/java/org/apache/baremaps/openstreetmap/utils/CRSUtils.java#L24