fix: Provide useful error message for empty or missing passwords for SCRAM auth

[sehrope]

Fixes #2288 to provide better error messages if the password is missing or empty during SCRAM auth.

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[sehrope]

Not sure why, but one of the two AppVeyor builds failed for the ScramTest, but it's for the test that I didn't touch: https://ci.appveyor.com/project/davecramer/pgjdbc/builds/41075794/job/nrrwrcsa5osywna1#L242

And the complaint is that the SQL state error code does not match the expected value:

org.opentest4j.AssertionFailedError: expected: <28P01> but was: <08001>
        at org.junit.jupiter.api.AssertionUtils.fail(AssertionUtils.java:55)
        at org.junit.jupiter.api.AssertionUtils.failNotEqual(AssertionUtils.java:62)
        at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:182)
        at org.junit.jupiter.api.AssertEquals.assertEquals(AssertEquals.java:177)
        at org.junit.jupiter.api.Assertions.assertEquals(Assertions.java:1124)
        at org.postgresql.jdbc.ScramTest.testPasswordWithoutSpace(ScramTest.java:99)

Not sure the cause but I'm guessing it's either some transient error on the CI server. I'm inclined to just ignore it for now and track down the cause for it later.

If anybody has any suggestions for improving the new error messages please let me know. Otherwise, I'll merge this in a few days.

[davecramer]

+1 I'd push this...