Better error message for AsdfFile.update in readonly mode

[drdavella]

This resolves #469.

Thanks to #579, this error should occur less often since by default asdf.open will attempt to determine the appropriate write mode for the file. In most cases, the default mode will be 'rw'.

[stsci-bot]

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[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

If there are any issues with this message, please report them here.

[coveralls]

Coverage remained the same at 93.684% when pulling 272fb94 on drdavella:better-error-msg into 414ad9d on spacetelescope:master.