Make it easier to find source of unexpected ability

[ceedubs]

I often encounter an error message like The expression in red needs the {IO} ability, but this location only has access to these abilities: {g5, Remote, Scope s8}. That's great (aside from some perplexing numbers appended to ability type parameters, but that's a separate issue).

But then when it shows me the offending code, it may be a block that is dozens of lines long. Is there a way that we could make it easier to spot where the offending code is?

This may be tricky, because there may be 1 or more nodes/lines requiring 1 or more unexpected abilities. Maybe the all could be highlighted? Even if it's easier to special-case 1 offending node/line, I think it would be pretty helpful.

Environment (please complete the following information):

• ucm --version 2d740de

[sellout]

Another message I see when I have this same needle-in-a-haystack problem is

When trying to match Mechanism {𝕖1953, 𝕖1954, Exception, Random} {𝕖1788, 𝕖1789, Random} with Mechanism {Random} {Random} the left hand side contained extra abilities: {𝕖1953, 𝕖1954, Exception}”

The message isn’t quite as good as the one @ceedubs sees, but I managed to interpret it as “You specified the type Mechanism {Random} {Random}, but the expression has the type Mechanism {Exception, Random} {Random}”.