UnitAbbreviationsCache: GetDefaultAbbreviation should throw when no abbreviations are mapped

[lipchev]

fixes the first part of #1509

• UnitAbbreviationsCache: GetDefaultAbbreviation should throw when no abbreviations are mapped
• Volume: added the missing abbreviation mappings

[lipchev]

This is missing a test for the unhappy path:

        [Fact]
        public void GetDefaultAbbreviation_ForUnitWithoutAbbreviations_ThrowsInvalidOperationException()
        {
            Assert.Throws<InvalidOperationException>(() => new UnitAbbreviationsCache([HowMuch.Info]).GetDefaultAbbreviation(HowMuchUnit.AShitTon));
        }

But this doesn't work just yet.. I'll add it later, probably after the little refactoring I got planned for the the UnitAbbreviationsCacheTests.