Improve error message for [build] structure errors

[perazz]

Address #889.
See discussion on the Fortran Discourse.

Solution: return package name on the error message triggered when an invalid [build] structure is detected.

John @urbanjost, can I ask you to give me a review of this change? (and edit the branch if you think this needs to be improved further). Thank you in advance.

[urbanjost]

Looks good. Wondering if it should be more verbose. Picture a user who is picking up an fpm package from someone else. The most extreme case is probably someone who knows nothing except he has been told to do an "fpm install". Should it explicitly say something like " syntax error in fpm.toml manifest file" ? If that occurred in the package fpm.toml file you would actually get an echo of the line where the error was found with a line number, such as

--> fpm.toml:28:2-6
|
13 | [build]
| ----- test

for most errors. Also, I think putting quotes around the keyword to indicate it is a literal string would be nice. It should be a rare problem; and getting the message that it is a dependent package is a good improvement. It is easy to envision a package with dozens of dependencies where this would help identify the problem immensely.

[perazz]

I agree. Here I've tried to improve the clarity of the output message a bit.

[urbanjost]

Yes, I think that is considerably more specific without being excessively verbose. Ideally we could try messages out on novices to see if they are clear even without familiarity with fpm; it is harder to judge when you have been using it for a while but I like it.

[perazz]

Thank you @urbanjost, I think this is good enough for now. I will merge, let's see if the users like it better than the previous form.