Improve error message for missing required parameters

[imagejan]

See #380 (comment).

Maybe we can add some API (to ObjectService? somewhere else?) to keep track of human-friendly types, such as:

String friendlyType = objectService.getHumanFriendlyName(Type type); // e.g. RandomAccessibleInterval => "an image"

(falling back to "an object"),
and then something like:

"This operation requires " + name + " of type " + type + "."

... to generate an error message such as:

This operation requires an image of type RandomAccessibleInterval.

[imagejan]

While looking into this, I was surprised to find that there is a different (slightly better?) message already, located in CheckInputsPreprocessor:

scijava-common/src/main/java/org/scijava/module/process/CheckInputsPreprocessor.java

Lines 56 to 58 in 3d64e2a

	if (input.isRequired() && input.getValue(module) == null) {
	cancel("'" + input.getName() + "' is required but unset.");
	}

... but it never/rarely gets executed because AbstractInputHarvester is taking it:

scijava-common/src/main/java/org/scijava/widget/AbstractInputHarvester.java

Lines 121 to 124 in 3d64e2a

	if (item.isRequired()) {
	throw new ModuleException("A " + type.getSimpleName() +
	" is required but none exist.");
	}