Add 9 more error explanations, improve some error messages.

[nham]

• Adds explanations for E0055, E0089, E0192, E0261-E0263, E0318
• Improves explanations for E0250, E0368, E0372.
• Converts 15 diagnostics to have error codes (E0380-E0394). Adds an explanation for E0380.
• The E0087-E0090 messages currently look like "expected {} parameter(s) found {} parameter(s)". This changes them to either use "parameter" or "parameters", based on the number.

This is, in part, more progress towards #24407

[rust-highfive]

r? @pcwalton

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[nham]

Of the new error codes (E0380-E0394), I've managed to trigger every error except E0385, E0388, and E0389.

By the way, it seems like E0385 might not be possible to trigger, because the AliasableOther variant doesn't seem to ever be constructed:

$ grep -r "AliasableOther" src/
src/librustc/middle/mem_categorization.rs:    AliasableOther,
src/librustc_borrowck/borrowck/mod.rs:            mc::AliasableOther => {

[alexcrichton]

@bors: r+ 412e549

Nice!

[bors]

☔ The latest upstream changes (presumably #25593) made this pull request unmergeable. Please resolve the merge conflicts.

[nham]

Rebased.

[alexcrichton]

@bors: r+

[bors]

📌 Commit 570a043 has been approved by alexcrichton

[bors]

⌛ Testing commit 570a043 with merge fe85342...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt