Tweak error messages and upload your own behavior.

[rebeccacremona]

In this PR, I'm doing three uncontroversial things, and one potentially controversial one.

Uncontroversial

1. While doing QA of the new Vue dashboard, I noticed that some of our error messages are unnecessarily terse or are otherwise a little clumsy. I tweaked them to suit my personal sensibilities; please feel free to suggest alternatives!

2. I noticed that the upload-your-own form wasn't handling titles and descriptions correctly: they should always be optional, and you should get the chance to supply them both when uploading fresh, or when replacing a failed capture.

3. I realized that I was wrong last week, when I said that the option to upload your own should not appear when you enter an invalid URL. That is in fact not correct: many valid URLs, like https://afdc.energy.gov/stations#/analyze?country=US&region=US-VA&fuel=ELEC, currently fail validation, and our users rely on this workaround. See LIL-836.

Potentially controversial

This PR significantly improved error handling.

But, in the process, it switched to unstyled messages... which I personally am finding difficult to parse, because the messages are long.

Here, I tried reintroducing a little styling.

I can easily revert it, if we prefer the unstyled, one-line version.

Examples:

[rebeccacremona]

(I also think it is a goofy we have two versions, a bunch that start with "We're unable to create your Perma Link" and a bunch that start with "Perma can't create this link." I am tempted to fix that up too... but am not sure.)

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 69.73%. Comparing base (cfda8c3) to head (8dc5720).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/api/serializers.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #3625   +/-   ##
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  Coverage    69.73%   69.73%           
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  Files           54       54           
  Lines         7326     7326           
========================================
  Hits          5109     5109           
  Misses        2217     2217           

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