handle FileNotFoundError in parse_requirements function

[mateuslatrova]

When the function "parse_requirements.txt" does not find the file passed as a parameter, it just raises an OSError.

This PR changes that behavior. Now, when that happens, this function handles it by printing a helpful message to the user and ends the program execution immediately.

[fredgrub]

This PR is related to PR https://github.com/bndr/pipreqs/pull/392. Since this PR modifies the functioning of the parse_requirements function, we must accept it first.

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (55eee29) 89.96% compared to head (4a28ae7) 90.07%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #397 +/- ## ========================================== + Coverage 89.96% 90.07% +0.11% ========================================== Files 2 2 Lines 259 262 +3 ========================================== + Hits 233 236 +3 Misses 26 26 ``` | [Files](https://app.codecov.io/gh/bndr/pipreqs/pull/397?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [pipreqs/pipreqs.py](https://app.codecov.io/gh/bndr/pipreqs/pull/397?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-cGlwcmVxcy9waXByZXFzLnB5) | `89.96% <87.23%> (+0.11%)` | :arrow_up: |

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[alan-barzilay]

LGTM just remove the white trailing space please

[alan-barzilay]

ah, and rebase, thx

[mateuslatrova]

@alan-barzilay rebased and fixed the white trailing space.