Improve error messages for incorrect file paths

[fedpet]

Running Alchemist with this parameter
-y "path/to/a/directory"
results in the following exception (at least on Windows 10):

14:55:55.543 [main] ERROR it.unibo.alchemist.Alchemist - Unable to load the requested file.
java.io.FileNotFoundException: path/to/a/directory (Access is denied)
	at java.base/java.io.FileInputStream.open0(Native Method)
	at java.base/java.io.FileInputStream.open(FileInputStream.java:213)
	at java.base/java.io.FileInputStream.<init>(FileInputStream.java:155)
	at it.unibo.alchemist.Alchemist.main(Alchemist.java:98)

Process finished with exit code 0

And, as suggested by @DanySK, a better error message along the lines of "I expected a file, but you provided a path to a directory" is needed