Add HTTPException with custom headers

[tiangolo]

Add HTTPException with custom headers and documentation for handling client errors.

[codecov]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #35   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          99    105    +6     
  Lines        2590   2687   +97     
=====================================
+ Hits         2590   2687   +97

Impacted Files	Coverage Δ
tests/test_starlette_exception.py	100% <100%> (ø)
fastapi/__init__.py	100% <100%> (ø)	⬆️
docs/src/handling_errors/tutorial002.py	100% <100%> (ø)
fastapi/applications.py	100% <100%> (ø)	⬆️
docs/src/security/tutorial003.py	100% <100%> (ø)	⬆️
..._tutorial/test_handling_errors/test_tutorial002.py	100% <100%> (ø)
docs/src/handling_errors/tutorial001.py	100% <100%> (ø)
..._tutorial/test_handling_errors/test_tutorial001.py	100% <100%> (ø)
fastapi/exceptions.py	100% <100%> (ø)
... and 5 more

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