Improve ctypes error reporting with missing DLL path

[pombredanne]

BPO	41836
Nosy	@pfmoore, @ericvsmith, @tjguk, @zware, @zooba, @The-Compiler, @pombredanne, @altendky

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-09-22.14:16:25.801>
created_at = <Date 2020-09-22.14:04:23.654>
labels = ['ctypes', 'type-feature']
title = 'Improve ctypes error reporting with missing DLL path'
updated_at = <Date 2020-09-22.14:16:25.800>
user = 'https://github.com/pombredanne'

bugs.python.org fields:

activity = <Date 2020-09-22.14:16:25.800>
actor = 'pombredanne'
assignee = 'none'
closed = True
closed_date = <Date 2020-09-22.14:16:25.801>
closer = 'pombredanne'
components = ['ctypes']
creation = <Date 2020-09-22.14:04:23.654>
creator = 'pombredanne'
dependencies = []
files = []
hgrepos = []
issue_num = 41836
keywords = []
message_count = 3.0
messages = ['377322', '377324', '377325']
nosy_count = 8.0
nosy_names = ['paul.moore', 'eric.smith', 'tim.golden', 'zach.ware', 'steve.dower', 'The Compiler', 'pombredanne', 'altendky']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41836'
versions = ['Python 3.6']

[pombredanne]

When the dependency of a DLL is missing (at least on Windows) the error " OSError: [WinError 126] The specified module could not be found" is raised when calling ctypes.CDLL(dll_path) even when this "dll_path" exists... because the error comes from another DLL.

These errors are really hard to diagnose because the path of the missing DLL is not returned in the exception message. Returning it would help fixing these kind of errors quickly.

Researching errors such as this one aboutcode-org/scancode-toolkit#2236 wastes quite a bit of time and would be made a non issue if we had the path in the error message.

[ericvsmith]

My understanding is that Windows doesn't tell you which DLL is missing. I think the best we could do is append something to the error message saying "or one its dependencies".

[pombredanne]

Eric, Thanks!
This is IMHO a dupe of https://bugs.python.org/issue25655 in earnest. So I am closing this in favor of that and will carry over comments there