Incomplete testing for format_date process plugin

@Jo Fitzgerald, good catch.

Let's add the destination property to this and then get this in. There is a suggestion in #2959444-Comment #10 to add a method on ProcessPluginBase to help with the error messages. But in the meantime just adding the destination property is helpful.

Ah, it was right in front of me, the destination property is there!

+++ b/core/modules/migrate/src/Plugin/migrate/process/FormatDate.php
@@ -136,7 +136,7 @@ public function transform($value, MigrateExecutableInterface $migrate_executable
+      throw new MigrateException(sprintf('Format date plugin could not transform %s value "%s" using the format "%s". Error: %s', $destination_property, $value, $fromFormat, $e->getMessage()), $e->getCode(), $e);

Nothing like a toddler to make me lose what I was trying to do here. I think the message needs tweaking. Saying that the plugin could not transform the destination property isn't quite right since the source value is transformed not the destination. Looking at the process plugins in the migrate for ideas there is only StaicMap that uses the destination_property in the error message.
throw new MigrateSkipRowException(sprintf("No static mapping found for '%s' and no default value provided for destination '%s'.", Variable::export($value), $destination_property));

Using that as a guide how about 'Format date plugin could not transform value "%s" using the format "%s" for destination '%s'. Error: %s' ?
What do you think?

@Jo Fitzgerald, excellent, thanks!

This adds a test for the exception message from FormatDate, which Jo Fitzgerald spotted and fixed, and the message includes more context which is always useful. There are plans to add even more context to the process plugin exception messages but that is still in the early stages and should not hold this up.

Committed 148b87f and pushed to 8.7.x. Thanks!