Improve error output for missing or malformed '-database' and '-source' parameters

[HaraldNordgren]

Related to #89

The current output only reads "no scheme" which is not really helpful.

Updates mattes/migrate#270
Updates mattes/migrate#317

[coveralls]

Pull Request Test Coverage Report for Build 164

• 17 of 17 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.02%) to 49.161%

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Totals
Change from base Build 160:	3.02%
Covered Lines:	850
Relevant Lines:	1729

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💛 - Coveralls

[HaraldNordgren]

Merge this @dhui @mattes?

[dhui]

I'm not sure if this is the correct place to validate parameters.
Do you have any examples of empty urls being used with migrate as a library?

If all of the confusion is with the migrate CLI, then we only need to validate the parameters in the CLI. e.g. ensure that source/path is specified

[HaraldNordgren]

I'm not sure. I haven't used the as a library yet. But I would imagine that usage like in godoc_vfs/vfs_example_test.go where we call

m, err := migrate.NewWithSourceInstance("godoc-vfs", d, "database://foobar")
...

would suffer from the same problems if database://foobar was replaced with something like os.Env("DATABASE_PATH") and that env variable ends up being empty for any reason. I'm making an assumption since I haven't tried this myself. But I assume that used when as a library it would still suffer from the same problems of unclear error messages.

[dhui]

env variable ends up being empty for any reason

Good point. Let's validate the URLs then.

[dhui]

Rename to errEmptyURL and use the string "URL cannot be empty".

Also, use errors.New() instead of fmt.Errorf() if string formatting isn't used.

[dhui]

Re-structure this to test valid URLs as well

[dhui]

rename to urlStr

[HaraldNordgren]

@dhui Updated.