provide better error message for prepare feature

[jjiwooLim]

Addressing OpenLiberty/ci.maven#1615.

Provide better error message when prepare-feature or prepareFeature task failed to generate features.json file for user feature(s).

In most cases, errors are caused by missing artifact properties in features-bom file (ex. <type>esa</type>) or wrong coordinates infeature-A.esa.pom, if it exists. If there is a pom file for the feature esa file, then it will read the pom file first and check if the esa exists in specified cooridinate. If the pom file doesn't exist, then it will directly check the esa file specified in features-bom file.

[jjiwooLim]

Updated error message:
If ESA file missing:

[INFO] --- liberty-maven-plugin:3.8.3-SNAPSHOT:prepare-feature (default-cli) @ liberty-maven-plugin-demo1 ---
[WARNING] org.apache.maven.plugin.MojoExecutionException: Unable to resolve artifact: test.user.test.osgi:SimpleActivatorESA:1.0.0-SNAPSHOT
[WARNING] The features.json could not be generated due to errors encountered while resolving the feature ESA file specified in feautres-bom file at coordinates test.user.test.osgi:features-bom:0.0.1-SNAPSHOT

If there's error in features-bom file:

[WARNING] Error: [type] tag(s) not found in features-bom file /Users/jiwoolim/.m2/repository/test/user/test/osgi/features-bom/0.0.1-SNAPSHOT/features-bom-0.0.1-SNAPSHOT.pom
[WARNING] The features.json could not be generated due to errors encountered while resolving the feature ESA file specified in feautres-bom file at coordinates test.user.test.osgi:features-bom:0.0.1-SNAPSHOT

If features-bom missing:

[ERROR] Failed to execute goal io.openliberty.tools:liberty-maven-plugin:3.8.3-SNAPSHOT:prepare-feature (default-cli) on project liberty-maven-plugin-demo1: Error preparing features.: org.apache.maven.plugin.MojoExecutionException: Unable to resolve artifact: test.user.test.osgi:features-bom:0.0.1-SNAPSHOT: Could not find artifact test.user.test.osgi:features-bom:pom:0.0.1-SNAPSHOT -> [Help 1]

[cherylking]

Please see the suggestion I made, which is what my previous comments were about. If it is not appropriate to issue that message when generatedJson does not exist, please add a comment to the code that explains why. Thanks.