Improve message when type resolution cannot deduce full record type

[julianhyde]

Improve message when type resolution cannot deduce full record type. Currently the expression

fn (e, job) => e.job = job

gives the following stack trace:

java.lang.ClassCastException: class net.hydromatic.morel.type.TypeVar cannot be cast to class net.hydromatic.morel.type.RecordLikeType (net.hydromatic.morel.type.TypeVar and net.hydromatic.morel.type.RecordLikeType are in unnamed module of loader 'app')
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:410)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:323)
	at net.hydromatic.morel.compile.Resolver.lambda$transform$0(Resolver.java:116)
	at com.google.common.collect.ImmutableList.forEach(ImmutableList.java:422)
	at net.hydromatic.morel.compile.Resolver.transform(Resolver.java:116)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:378)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:335)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:405)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:323)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:593)
	at net.hydromatic.morel.compile.Resolver.lambda$transform$0(Resolver.java:116)
	at com.google.common.collect.ImmutableList.forEach(ImmutableList.java:422)
	at net.hydromatic.morel.compile.Resolver.transform(Resolver.java:116)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:440)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:325)
	at net.hydromatic.morel.compile.Resolver.lambda$resolveValDecl$6(Resolver.java:201)
	at java.base/java.util.LinkedHashMap.forEach(LinkedHashMap.java:684)
	at net.hydromatic.morel.compile.Resolver.resolveValDecl(Resolver.java:199)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:144)
	at net.hydromatic.morel.compile.Resolver.toCore(Resolver.java:123)
	at net.hydromatic.morel.compile.Compiles.prepareDecl(Compiles.java:89)
	at net.hydromatic.morel.compile.Compiles.prepareStatement(Compiles.java:71)

It should instead give the following error:

unresolved flex record (can't tell what fields there are besides #job)

As part of this change, we introduce class TypeResolver.TypeException, and add the method Ml.assertTypeException(String message) to improve the testing of types.