Return a more descriptive error when provided mapping file does not exist

[DylanVanAssche]

Description

RML Mapper crashes with a parsing error for non-existing mapping files:

08:34:42.569 [main] ERROR be.ugent.rml.cli.Main               .main(315) - Unexpected end of file
org.eclipse.rdf4j.rio.RDFParseException: Unexpected end of file
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.throwEOFException(TurtleParser.java:1317)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.verifyCharacterOrFail(TurtleParser.java:1133)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parseQNameOrBoolean(TurtleParser.java:1001)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parseValue(TurtleParser.java:578)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parseSubject(TurtleParser.java:406)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parseTriples(TurtleParser.java:347)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parseStatement(TurtleParser.java:216)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parse(TurtleParser.java:178)
	at org.eclipse.rdf4j.rio.turtle.TurtleParser.parse(TurtleParser.java:130)
	at be.ugent.rml.store.RDF4JStore.read(RDF4JStore.java:111)
	at be.ugent.rml.cli.Main.main(Main.java:174)
	at be.ugent.rml.cli.Main.main(Main.java:35)

I find it useful to check if the mapping file exists before parsing it. This way, we can return an appropriate error message.

Steps to reproduce

1. Execute the RML Mapper with -m <mapping file>. The name of the mapping file is a non-existing.
2. The RML Mapper fails with a Turtle parsing error.

Environment

• Ubuntu 18.04 LTS
• OpenJDK 12.0.2 (64 bit)
• RMLMapper 4.7.0