Refactor run.sh: added custom build-plan file and build plan checks; improved messaging

[OnkelTem]

See: be5invis/Iosevka#1340

This PR adds some messaging and checks.

From the very beginning of the run.sh script:

1. It prints which build-plans file it's gonna use
2. It prints which build plan it's gonna use
3. It doesn't dismiss the build plan anymore when there is no custom build-plains file. It's getting passed down to npm run build.

Sample session, without custom build-plans file and build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build
Custom build-plans file not found, using the default one
Using build plan: contents::iosevka
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

With custom build-plans file, but without a build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build
Found custom build-plans file: private-build-plans.toml
Using build plan: contents::iosevka-sans-q
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

With custom build-plans file and custom build plan:

$ docker run -it -v $(pwd):/build avivace/iosevka-build ttf::iosevka-sans-q
Found custom build-plans file: private-build-plans.toml
Using build plan: ttf::iosevka-sans-q
Using font version: 15.2.0
Downloading and checking the validity of the source code...
...

[OnkelTem]

Wait, I found a mistake

[OnkelTem]

I'm thinking of a case when the above logic would prevent people from using the default build-plans file with custom build param. Currently the script would fail. Not sure if it's ok. Let's remove it. After all, if custom build plan is wrong, it will fail later at npm run build...

[avivace]

Yes, I guess you're right.. but what would be bad about leaving the error message?

[OnkelTem]

But maybe that the user knows what they're doing. For example, now it's possible to pass the param directly to the npm script. E.g to build only TTFs with the rest by default:

$ docker run -it -v $(pwd):/build avivace/iosevka-build ttf::iosevka

[nobk]

iosevka-build can use like this, but you patch can not.
iosevka-build ttf::my_font1 ttf::my_font2 ttf::my_font3 ttf::my_font4 ttf::my_font5

[nobk]

@avivace $1 should change to $@ to support several fonts build with in one pass.