Provide a more informative error when the `checksum` fails onLoad of FeatureExtraction

[anthonysena]

When the Java library for FeatureExtraction is updated, you must update the inst/csv/jarChecksum.txt which is done through:

FeatureExtraction/extras/PackageMaintenance.R

Lines 86 to 88 in 437570a

	# Store JAR checksum --------------------------------------------------------------
	checksum <- rJava::J("org.ohdsi.featureExtraction.JarChecksum", "computeJarChecksum")
	write(checksum, file.path("inst", "csv", "jarChecksum.txt"))

If you do not update the inst/csv/jarChecksum.txt, you get the following error:

FeatureExtraction/R/FeatureExtraction.R

Lines 42 to 46 in 437570a

	if (computedChecksum != "" && (storedChecksum != computedChecksum)) {
	warning("Java library version does not match R package version! Please try reinstalling the FeatureExtraction package.
	Make sure to close all instances of R, and open only one instance before reinstalling. Also make sure your
	R workspace is not reloaded on startup. Delete your .Rdata file if necessary")
	}

It would be useful to update this description to include some information for Java developers to indicate that the origin of this error lies with a mismatch between the inst/csv/jarChecksum.txt and current state of the FE Java library.