CI: Fixes to the patterns code checks
Fixing couple of small errors in the CI (detected in https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=5707):
- When the `patterns` (grep unwanted patterns) step fails, the installation of flake8... is cancelled
- When a file has a blank line with whitespaces, the reported error is not clear

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=h1) Report
> Merging [#24410](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fc7bc3f74d1f5702bf66c519ad190126538a8f5b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24410/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24410      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51943    51943              
==========================================
+ Hits        47946    47947       +1     
+ Misses       3997     3996       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24410/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=footer). Last update [fc7bc3f...9a0e321](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=h1) Report
> Merging [#24410](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/fc7bc3f74d1f5702bf66c519ad190126538a8f5b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24410/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24410      +/-   ##
==========================================
+ Coverage    92.3%    92.3%   +<.01%     
==========================================
  Files         163      163              
  Lines       51943    51943              
==========================================
+ Hits        47946    47947       +1     
+ Misses       3997     3996       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `42.99% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24410/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.84% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=footer). Last update [fc7bc3f...9a0e321](https://codecov.io/gh/pandas-dev/pandas/pull/24410?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @datapythonista 