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]