LinkageCheckerMain to provide clear error message when the supplied coordinates are invalid

When I pass an invalid coordinates to LinkageCheckerMain, it throws `IndexOutOfBoundsException`. This does not tell the cause of the error.

```
> Task :checkJavaLinkage
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
Exception in thread "main" java.lang.IndexOutOfBoundsException: end index (1) must not be greater than size (0)
at com.google.common.base.Preconditions.checkPositionIndexes(Preconditions.java:1420)
at com.google.common.collect.ImmutableList.subList(ImmutableList.java:433)
at com.google.cloud.tools.opensource.classpath.LinkageCheckerMain.checkArtifacts(LinkageCheckerMain.java:159)
at com.google.cloud.tools.opensource.classpath.LinkageCheckerMain.main(LinkageCheckerMain.java:66)

> Task :checkJavaLinkage FAILED

```

Start in LinkageCheckerMain.checkArtifacts at line 159 and reproduce the failure with invalid coordinates. Trace how the supplied coordinates are handled, then verify that invalid input produces a clear error message instead of IndexOutOfBoundsException.

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