Add `Builder::try_parse()` method

[Maximkaaa]

Current implementation of the Builder::parse() method prints out specification errors to stderr and then just ignores them. This is fine for most console applications, but in some cases better control over what is happening is needed:

• Sometimes there is no access to stderr, in that case there is no way to find out what went wrong
• For some applications it's more desirable to fail on startup than to run with (partially) invalid configuration.

For such cases this PR introduces a new method try_parse that does the same thing as the parse method, but returns an Err in case an error in the specification is found.

This change was suggested in and fixes #323.

[coveralls]

Pull Request Test Coverage Report for Build 10060325859

Details

• 12 of 40 (30.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-1.0%) to 42.535%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
crates/env_filter/src/filter.rs	5	12	41.67%
crates/env_filter/src/parser.rs	7	28	25.0%
<!--	Total:	12	40	30.0%	-->

Totals
Change from base Build 9504345986:	-1.0%
Covered Lines:	245
Relevant Lines:	576

---

💛 - Coveralls

[epage]

Thanks for moving this forward!

[Maximkaaa]

I fixed all the comments except the last one, since I think that the current approach is better. If you insist on returning just one error string, I will edit it also.

[epage]

Looks like the final commit needs cargo fmt run on it

[Maximkaaa]

Looks like the final commit needs cargo fmt run on it

Done.

[epage]

Thanks! This is now released.