Show symbol index that caused an error

[SomeoneToIgnore]

When the line parsed with an error is long, say DATABASE_URL="postgres://${POSTGRES_USER}:${POSTGRES_PASSWORD}@localhost:5432/$\{POSTGRES_DB}"

it's rather hard to understand which symbol had caused a parsing error.

This PR makes it a bit more clearer. It would be even more descriptive to actually highlight the wrong symbol instead with some symbol, say ^, but not sure if it's a good approach for the library crate.

[codecov[bot]]

Codecov Report

Merging #7 into master will increase coverage by 0.32%. The diff coverage is 88.88%.

@@           Coverage Diff           @@
##           master    #7      +/-   ##
=======================================
+ Coverage   90.67%   91%   +0.32%     
=======================================
  Files          18    18              
  Lines         547   567      +20     
=======================================
+ Hits          496   516      +20     
  Misses         51    51

Impacted Files	Coverage Δ
dotenv_codegen_implementation/src/lib.rs	0% <0%> (ø)	:arrow_up:
dotenv/src/errors.rs	90.47% <80%> (ø)	:arrow_up:
dotenv/src/parse.rs	97.41% <93.33%> (-0.59%)	:arrow_down:
dotenv/tests/test-from-filename-iter.rs	90% <0%> (ø)	:arrow_up:
dotenv/src/lib.rs	100% <0%> (ø)	:arrow_up:
dotenv/tests/test-dotenv-iter.rs	90% <0%> (ø)	:arrow_up:
dotenv/tests/test-from-path-iter.rs	91.66% <0%> (ø)	:arrow_up:

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[SomeoneToIgnore]

I don't understand the codecov status, is it stating that the PR had increased the coverage but it's not enough still?

Looks rather unfair to me.

[ZoeyR]

It's just saying that of the code changed 88% was covered whereas the target is 90%. There is a reason I don't have codecov listed as a required status.