[Policy Compiler] Surface type-mismatches error in a more readable fashion

[l46kok]

Policy compiler will fail to type-check the composed policies (expectedly) if their output types are different. Example:

name: foo_policy
rule:
    match:
       - condition: x
         output: "error"
       - output: true

Compile() failed: ERROR: testdata/nested_rule/policy.yaml:-1:0: found no matching overload for '_?_:_' applied to '(bool, map(string, bool), string)'

We should try to raise this error in a more readable fashion, and ideally with the source positions wrt/ yaml.

(xref b/350064634 for Java impl)