Improve an error message when failed to load pyproject.toml

[thombashi]

This PR aims to improve the information content of an error message when failed to load pyproject.toml file.

Like #637, current black 18.9b0 (and master branch) output the following error message when pyproject.toml malformed or toml package considers the format is invalid:

$ black .
Usage: black [OPTIONS] [SRC]...

Error: <click.core.Context object at 0x7f51c84264e0>

I think that it is difficult to understand the cause of the error from the message.
After applying the patch, the message looks like below:

$ black .
Error: Could not open file /home/.../pyproject.toml: Error reading configuration file: Invalid date or number (line 36 column 1 char 478)

[coveralls]

Pull Request Test Coverage Report for Build 850

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	0	1	0.0%

Totals
Change from base Build 838:	0.0%
Covered Lines:	2775
Relevant Lines:	3029

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 850

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	0	1	0.0%

Totals
Change from base Build 838:	0.0%
Covered Lines:	2775
Relevant Lines:	3029

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 850

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	0	1	0.0%

Totals
Change from base Build 838:	0.0%
Covered Lines:	2775
Relevant Lines:	3029

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 850

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	0	1	0.0%

Totals
Change from base Build 838:	0.0%
Covered Lines:	2775
Relevant Lines:	3029

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 850

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 91.614%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
black.py	0	1	0.0%

Totals
Change from base Build 838:	0.0%
Covered Lines:	2775
Relevant Lines:	3029

---

💛 - Coveralls

[JelleZijlstra]

Thank you!