Add annotation on lambdas to improve type error locations

[codecov-commenter]

Codecov Report

Patch coverage: 93.75% and project coverage change: +0.08% :tada:

Comparison is base (ae52f82) 91.73% compared to head (fc99ca2) 91.82%. Report is 1 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1038 +/- ## ========================================== + Coverage 91.73% 91.82% +0.08% ========================================== Files 91 91 Lines 9479 9487 +8 Branches 2197 2172 -25 ========================================== + Hits 8696 8711 +15 + Misses 783 776 -7 ``` | [Files Changed](https://app.codecov.io/gh/johnynek/bosatsu/pull/1038?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin) | Coverage Δ | | |---|---|---| | [core/src/main/scala/org/bykn/bosatsu/Pattern.scala](https://app.codecov.io/gh/johnynek/bosatsu/pull/1038?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin#diff-Y29yZS9zcmMvbWFpbi9zY2FsYS9vcmcvYnlrbi9ib3NhdHN1L1BhdHRlcm4uc2NhbGE=) | `96.00% <66.66%> (-0.23%)` | :arrow_down: | | [.../main/scala/org/bykn/bosatsu/SourceConverter.scala](https://app.codecov.io/gh/johnynek/bosatsu/pull/1038?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin#diff-Y29yZS9zcmMvbWFpbi9zY2FsYS9vcmcvYnlrbi9ib3NhdHN1L1NvdXJjZUNvbnZlcnRlci5zY2FsYQ==) | `97.58% <100.00%> (+0.02%)` | :arrow_up: | ... and [7 files with indirect coverage changes](https://app.codecov.io/gh/johnynek/bosatsu/pull/1038/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=P.+Oscar+Boykin)

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