User input errors in DashboardMain should not create stack traces

All possible user input errors should be handled and messaged without exposing exceptions

[WARNING] java.lang.IllegalArgumentException: The versionless coordinates should have one colon character: com.google.cloud:libraries-bom:1.0.0 at com.google.common.base.Preconditions.checkArgument (Preconditions.java:142) at com.google.cloud.tools.opensource.dashboard.DashboardMain.generateAllVersions (DashboardMain.java:119) at com.google.cloud.tools.opensource.dashboard.DashboardMain.main (DashboardMain.java:107) at org.codehaus.mojo.exec.ExecJavaMojo$1.run (ExecJavaMojo.java:254) at java.lang.Thread.run (Thread.java:748) [INFO] ------------------------------------------------------------------------ [INFO] BUILD FAILURE [INFO] ------------------------------------------------------------------------ [INFO] Total time: 0.659 s [INFO] Finished at: 2020-07-13T10:05:37-04:00 [INFO] ------------------------------------------------------------------------ [ERROR] Failed to execute goal org.codehaus.mojo:exec-maven-plugin:3.0.0:java (default-cli) on project dashboard: An exception occured while executing the Java class. The versionless coordinates should have one colon character: com.google.cloud:libraries-bom:1.0.0 -> [Help 1] [ERROR]