Improve type error message in cypher `reduce(...)`

For the case where the accumulator and the expression have mismatching
types, it was not clear that the type error was for that reason. This
change improves the message for that case.
Merge pull request #2806 from cleishm/cypher-reduce-type-error

Improve type error message in cypher `reduce(...)`
Merge remote-tracking branch 'upstream/2.0-maint' into 2.1-maint

Conflicts:
	community/cypher/cypher-compiler-2.1/src/main/scala/org/neo4j/cypher/internal/compiler/v2_1/ast/IterableExpressions.scala
	community/cypher/cypher-compiler-2.1/src/test/scala/org/neo4j/cypher/internal/compiler/v2_1/ast/ExpressionTest.scala
	community/cypher/cypher-compiler-2.1/src/test/scala/org/neo4j/cypher/internal/compiler/v2_1/ast/ReduceExpressionTest.scala
	community/cypher/cypher/src/test/scala/org/neo4j/cypher/SemanticErrorAcceptanceTest.scala
Merge pull request #2812 from systay/2.0-into-2.1

Merge 2.0 into 2.1